The scientific results of the first three years of observing the cosmos with Des, the Dark Energy Survey, were presented on May 27th. Results in agreement with the predictions of the standard model of cosmology, albeit with an indication of possible discrepancy regarding the “graininess” of the universe
Over four hundred female scientists from seven countries. 758 nights of telescope observations, distributed between 2013 and 2019. Hundreds of millions of objects cataloged. Objective: To produce the most accurate description ever of seven billion years of universe history. An impressive effort, that of the Dark Energy Survey (Des) observing campaign , which culminated last May 27 in the presentation of 29 scientific articles with the results of the first three years of observations. Results that substantially confirm the validity of the standard cosmological model (the so – called Lambda-Cdm model , the one with dark energy and dark matter, so to speak), albeit showing a graininess ( clumpiness, in English) slightly lower than expected, as if today’s universe were a few percentage points more uniform – therefore with fewer “lumps” – than predicted based on observations of the early universe.
And it is precisely from the early universe – and in particular from the “photograph” of the cosmos 380 thousand years after the Big Bang, immortalized with great precision in the maps of the Planck space telescope – that the Des survey started . If we take the universe back then (therefore over 13 billion years ago) as a starting point and apply the cosmological model to predict its evolution to the present day, the scientists of Des said to themselves, then comparing the result with the observations of the survey we will be able to evaluate the goodness or otherwise of the cosmological model itself. Observations that were conducted with the Dark Energy Camera – a 570 megapixel digital camera mounted on the Blanco Telescope– with the aim of producing the most extensive and precise maps ever obtained of the distribution of galaxies in the universe in relatively recent times. We are talking about 226 million galaxies observed in 345 nights, one hundred million of which were then used for cosmological analysis.
«The published results show compatibility between Des’s predictions and measures. However, some elements of Des’s analysis and other competing experiments continue to suggest a slight tension between the observations of the nearby universe and those of the primitive universe “, explains referring to the excess of uniformity from which we mentioned earlier Marco Raveri , postdoc researcherto UPenn – the University of Pennsylvania at Philadelphia – who led the statistical analysis on this discrepancy. A discrepancy, the authors of the study underline, is not large enough to exclude that it is only a statistical fluctuation: the analysis of the remaining half of Des data, expected for the next few years, will be important to try to arrive at an answer definitive.
The methods used by Des’s team to trace the distribution of dark matter and quantify the effect of dark energy are mainly based on the observation of two effects due to the gravitational attraction of dark matter on normal matter and on light. The first is the large-scale distribution of galaxy clusters – a cosmic web that traces the density of the underlying dark matter. The second is weak gravitational lensing , or the slight but statistically significant distortion of the shape of objects in the background introduced by the presence of large masses – mainly dark matter – between them and us who observe them.
“By analyzing the subtle distortions on our one hundred million galaxies, Des was able to trace the distribution of matter that produces them”, says Marco Gatti , also a postdoc researcher at UPenn, at the head of the group that elaborates the maps. of matter. “These are the largest matter maps ever created: they cover an eighth of the sky and show, above all, dark matter, which does not emit light and cannot be detected with traditional methods.”
During the survey, the Dark Energy Camera of Des has repeatedly focused on ten regions of the sky selected to obtain as many “deep fields”, so as to be able to glimpse even the most distant galaxies. The measurement of the redshift – therefore of the distance – of these remote galaxies was then used to precisely calibrate the rest of the survey .
“A key point was the development of new methodologies to measure the redshift of one hundred million galaxies, which makes it possible to produce a 3D map of the universe”, observes one of those responsible for these measurements, Giulia Giannini , PhD researcher at IFAE, the Barcelona Institute of High Energy Physics. “Various independent methods have been combined, applying advanced statistical techniques, more sophisticated and precise than those adopted so far, to characterize the relationship between color and position of galaxies and their redshift with the greatest possible accuracy, which is fundamental to avoid obtaining false results “.
The next step will come with the results of the data analysis of the remaining three years of survey, and should lead to an even more accurate picture of dark matter and dark energy in the universe.
The largest ever map of dark matter – invisible matter thought to account for 80% of the total matter of the Universe – has been created by a team co-led by UCL researchers, as part of the international Dark Energy Survey (DES).
As matter curves space-time, astronomers are able to map its existence by looking at light travelling to Earth from distant galaxies; if the light has been distorted, this means there is matter in the foreground, bending the light as it comes towards us.
The team used artificial intelligence methods to analyse images of 100 million galaxies, looking at their shape – spots of light made up of 10 or so pixels – to see if they have been stretched.
The new map, a representation of all matter detected in the foreground of the observed galaxies, covers a quarter of the sky of the Southern Hemisphere. It is described in a new paper posted on the DES website and to be published in the Monthly Notices of the Royal Astronomical Society.
Co-lead author Dr Niall Jeffrey (both at École Normale Supérieure, Paris, and UCL Physics & Astronomy) said: “Most of the matter in the Universe is dark matter. It is a real wonder to get a glimpse of these vast, hidden structures across a large portion of the night sky. These structures are revealed using the distorted shapes of hundreds of millions of distant galaxies with photographs from the Dark Energy Camera in Chile.
“In our map, which mainly shows dark matter, we see a similar pattern as we do with visible matter only– a web-like structure with dense clumps of matter separated by large empty voids.
“Observing these cosmic-scale structures can help us to answer fundamental questions about the Universe.”
Since the 1930s, astronomers have suspected there is more material in the Universe than we can see. Dark matter, like dark energy, remains mysterious, but its existence is inferred from galaxies behaving in ways not predicted – for instance, the fact that galaxies stay clustered together, and that galaxies within clusters move faster than expected.
Co-author Professor Ofer Lahav (UCL Physics & Astronomy), chair of the DES UK consortium and co-Director of UCL’s Centre for Doctoral Training Centre in Data Intensive Science, said: “Visible galaxies form in the densest regions of dark matter. When we look at the night sky, we see the galaxy’s light but not the surrounding dark matter – like looking at the lights of a city at night.
“By calculating how gravity distorts light, a technique known as gravitational lensing, we get the whole picture – both visible and invisible matter. This brings us closer to understanding what the Universe is made of and how it has evolved. It also shows the power of artificial intelligence methods to analyse one of the largest data sets in astronomy.”
Co-author Dr Chihway Chang (University of Chicago) said: “Our map projects 3D space, extending out over seven billion light years, into a 2D representation. In our next phase of work, we will analyse tomographic maps at varying distances to build a 3D view of the Universe. These maps will also enhance our understanding of the connection between dark matter and galaxies.”
The map was created following an immense amount of careful work to measure the galaxy shapes by the DES collaboration, hosted by the US Department of Energy’s Fermi National Accelerator Laboratory (Fermilab) and involving more than 400 scientists from 25 institutions in seven nations.
The international project aims to measure the composition and growth of the Universe and to better understand dark matter and dark energy, which is thought to be driving the Universe’s accelerating expansion.
The collaboration has catalogued hundreds of millions of galaxies, using photographs of the night sky taken by the 570-megapixel Dark Energy Camera, one of the world’s most powerful digital cameras, over six years (from 2013 to 2019). The camera is mounted on a telescope at the National Science Foundation’s Cerro Tololo Inter-American Observatory in Chile. Dr Marco Gatti, of the University of Pennsylvania, a co-lead author of the dark matter paper, said: “These images provide us with the most precise ever look at the Universe’s evolution over the last seven billion years.”
New analysis of the first three years of the survey by DES scientists suggests that matter is distributed throughout the Universe in a way that is consistent with predictions in the standard cosmological model, the best current model of the Universe.
But the analysis found hints, as with previous surveys, that the Universe may be a few per cent smoother than predicted. This prediction comes from analysis of the light left over from the Big Bang (known as the cosmic microwave background, or CMB).
Dr Pablo Lemos (both UCL Physics & Astronomy and Sussex University), co-author of the new analysis paper, said: “It would be very exciting to find contradictions between galaxy surveys like DES and analyses of the CMB, as they would provide hints of new physics. This observed difference in the clustering of matter could be one such contradiction, but we will need more data to confirm it.”
Current evidence obtained through analysing the CMB suggests the Universe at present is made up of approximately 5% ordinary, visible matter, 25% dark matter, and 70% dark energy. DES Director and spokesperson Professor Rich Kron, who is a Fermilab and University of Chicago scientist, said: “DES seeks to illuminate the natures of dark matter and dark energy by studying how the competition between them shapes the large-scale structure of the Universe over cosmic time.”
In the UK the study received funding from the Science and Technology Facilities Council (STFC) and theHigher Education Funding Council for England. Other funding for DES projects has been provided by the U.S. Department of Energy, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, Funding Authority for Funding and Projects in Brazil, Carlos Chagas Filho Foundation for Research Support of the State of Rio de Janeiro, the Brazilian National Council for Scientific and Technological Development and the German Research Foundation.
Reference: Niall Jeffrey, Justin Alsing, François Lanusse, Likelihood-free inference with neural compression of DES SV weak lensing map statistics, Monthly Notices of the Royal Astronomical Society, Volume 501, Issue 1, February 2021, Pages 954–969, https://doi.org/10.1093/mnras/staa3594
First three years of survey data uses observations of 226 million galaxies over ⅛ of the sky
In 29 new scientific papers, the Dark Energy Survey examines the largest-ever maps of galaxy distribution and shapes, extending more than 7 billion light-years across the Universe. The extraordinarily precise analysis, which includes data from the survey’s first three years, contributes to the most powerful test of the current best model of the Universe, the standard cosmological model. However, hints remain from earlier DES data and other experiments that matter in the Universe today is a few percent less clumpy than predicted.
New results from the Dark Energy Survey (DES) use the largest-ever sample of galaxies observed over nearly one-eighth of the sky to produce the most precise measurements to date of the Universe’s composition and growth.
DES images the night sky using the 570-megapixel Dark Energy Camera on the National Science Foundation’s Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile, a Program of NSF’s NOIRLab. One of the most powerful digital cameras in the world, the Dark Energy Camera was designed specifically for DES. It was funded by the Department of Energy (DOE) and was built and tested at DOE’s Fermilab.
Over the course of six years, from 2013 to 2019, DES used 30% of the time on the Blanco Telescope and surveyed 5000 square degrees — almost one-eighth of the entire sky — in 758 nights of observation, cataloging hundreds of millions of objects. The results announced today draw on data from the first three years — 226 million galaxies observed over 345 nights — to create the largest and most precise maps yet of the distribution of galaxies in the Universe at relatively recent epochs. The DES data were processed at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign.
“NOIRLab is a proud host for and member of the DES collaboration,” said Steve Heathcote, CTIO Associate Director. “Both during and after the survey, the Dark Energy Camera has been a popular choice for community and Chilean astronomers.”
At present the Dark Energy Camera is used for programs covering a huge range of science including cosmology. The Dark Energy Camera science archive, including DES Data Release 2 on which these results are based, is curated by the Community Science and Data Center (CSDC), a Program of NSF’s NOIRLab. CSDC provides software systems, user services, and development initiatives to connect and support the scientific missions of NOIRLab’s telescopes, including the Blanco telescope at CTIO.
Since DES studied nearby galaxies as well as those billions of light-years away, its maps provide both a snapshot of the current large-scale structure of the Universe and a view of how that structure has evolved over the past 7 billion years.
Ordinary matter makes up only about 5% of the Universe. Dark energy, which cosmologists hypothesize drives the accelerating expansion of the Universe by counteracting the force of gravity, accounts for about 70%. The last 25% is dark matter, whose gravitational influence binds galaxies together. Both dark matter and dark energy remain invisible. DES seeks to illuminate their nature by studying how the competition between them shapes the large-scale structure of the Universe over cosmic time.
To quantify the distribution of dark matter and the effect of dark energy, DES relied mainly on two phenomena. First, on large scales galaxies are not distributed randomly throughout space but rather form a weblike structure that is due to the gravity of dark matter. DES measured how this cosmic web has evolved over the history of the Universe. The galaxy clustering that forms the cosmic web in turn revealed regions with a higher density of dark matter.
Second, DES detected the signature of dark matter through weak gravitational lensing. As light from a distant galaxy travels through space, the gravity of both ordinary and dark matter in the foreground can bend its path, as if through a lens, resulting in a distorted image of the galaxy as seen from Earth. By studying how the apparent shapes of distant galaxies are aligned with each other and with the positions of nearby galaxies along the line of sight, DES scientists were able to infer the clumpiness of the dark matter in the Universe.
To test cosmologists’ current model of the Universe, DES scientists compared their results with measurements from the European Space Agency’s orbiting Planck observatory. Planck used light known as the cosmic microwave background to peer back to the early Universe, just 400,000 years after the Big Bang. The Planck data give a precise view of the Universe 13 billion years ago, and the standard cosmological model predicts how the dark matter should evolve to the present.
Combined with earlier results DES provides the most powerful test of the current best model of the Universe to date, and the results are consistent with the predictions of the standard model of cosmology. However, hints remain from DES and several previous galaxy surveys that the Universe today is a few percent less clumpy than predicted .
Ten regions of the sky were chosen as “deep fields” that the Dark Energy Camera imaged repeatedly throughout the survey. Stacking those images together allowed the scientists to glimpse more distant galaxies. The team then used the redshift information from the deep fields to calibrate the rest of the survey region. This and other advancements in measurements and modeling, coupled with a threefold increase in data compared to the first year, enabled the team to pin down the density and clumpiness of the Universe with unprecedented precision.
DES concluded its observations of the night sky in 2019. With the experience gained from analyzing the first half of the data, the team is now prepared to handle the complete dataset. The final DES analysis is expected to paint an even more precise picture of the dark matter and dark energy in the Universe.
The DES collaboration consists of over 400 scientists from 25 institutions in seven countries.
“The collaboration is remarkably young. It’s tilted strongly in the direction of postdocs and graduate students who are doing a huge amount of this work,” said DES Director and spokesperson Rich Kron, who is a Fermilab and University of Chicago scientist. “That’s really gratifying. A new generation of cosmologists are being trained using the Dark Energy Survey.”
The methods developed by the team have paved the way for future sky surveys such as the Rubin Observatory Legacy Survey of Space and Time. “DES shows that the era of big survey data has well and truly begun,” notes Chris Davis, NSF’s Program Director for NOIRLab. “DES on NSF’s Blanco telescope has set the scene for the remarkable discoveries to come with Rubin Observatory over the coming decade.”
The recent DES results will be presented in a scientific seminar on 27 May 2021. Twenty-nine papers are available on the arXiv online repository and from the Dark Energy Survey website. The main paper is Dark Energy Survey Year 3 Results: Cosmological Constraints from Galaxy Clustering and Weak Lensing.
NSF’s NOIRLab (National Optical-Infrared Astronomy Research Laboratory), the US center for ground-based optical-infrared astronomy, operates the international Gemini Observatory (a facility of NSF, NRC–Canada, ANID–Chile, MCTIC–Brazil, MINCyT–Argentina, and KASI–Republic of Korea), Kitt Peak National Observatory (KPNO), Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), and Vera C. Rubin Observatory (in cooperation with DOE’s SLAC National Accelerator Laboratory). It is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona. The astronomical community is honored to have the opportunity to conduct astronomical research on Iolkam Du’ag (Kitt Peak) in Arizona, on Maunakea in Hawaiʻi, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence that these sites have to the Tohono O’odham Nation, to the Native Hawaiian community, and to the local communities in Chile, respectively.
The Dark Energy Survey is a collaboration of more than 400 scientists from 25 institutions in seven countries. For more information about the survey, please visit the experiment’s website.
Funding for the DES Projects has been provided by the US Department of Energy, the US National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, Funding Authority for Funding and Projects in Brazil, Carlos Chagas Filho Foundation for Research Support of the State of Rio de Janeiro, Brazilian National Council for Scientific and Technological Development and the Ministry of Science and Technology, the German Research Foundation and the collaborating institutions in the Dark Energy Survey.
Featured image: Ten areas in the sky were selected as “deep fields” that the Dark Energy Camera imaged several times during the survey, providing a glimpse of distant galaxies and helping determine their 3D distribution in the cosmos. The image is teeming with galaxies — in fact, nearly every single object in this image is a galaxy. Some exceptions include a couple of dozen asteroids as well as a few handfuls of foreground stars in our own Milky Way. Credit: Dark Energy Survey/DOE/FNAL/DECam/CTIO/NOIRLab/NSF/AURA
Reference: C Doux, E Baxter, P Lemos, C Chang, A Alarcon, A Amon, A Campos, A Choi, M Gatti, D Gruen, M Jarvis, N MacCrann, Y Park, J Prat, M M Rau, M Raveri, S Samuroff, J DeRose, W G Hartley, B Hoyle, M A Troxel, J Zuntz, T M C Abbott, M Aguena, S Allam, J Annis, S Avila, D Bacon, E Bertin, S Bhargava, D Brooks, D L Burke, M Carrasco Kind, J Carretero, R Cawthon, M Costanzi, L N da Costa, M E S Pereira, S Desai, H T Diehl, J P Dietrich, P Doel, S Everett, I Ferrero, P Fosalba, J Frieman, J García-Bellido, D W Gerdes, T Giannantonio, R A Gruendl, J Gschwend, G Gutierrez, S R Hinton, D L Hollowood, K Honscheid, E M Huff, D Huterer, B Jain, D J James, E Krause, K Kuehn, N Kuropatkin, O Lahav, C Lidman, M Lima, M A G Maia, F Menanteau, R Miquel, R Morgan, J Muir, R L C Ogando, A Palmese, F Paz-Chinchón, A A Plazas, E Sanchez, V Scarpine, M Schubnell, S Serrano, I Sevilla-Noarbe, M Smith, E Suchyta, M E C Swanson, G Tarle, C To, D L Tucker, T N Varga, J Weller, R D Wilkinson, (DES Collaboration), Dark energy survey internal consistency tests of the joint cosmological probes analysis with posterior predictive distributions, Monthly Notices of the Royal Astronomical Society, Volume 503, Issue 2, May 2021, Pages 2688–2705, https://doi.org/10.1093/mnras/stab526
Matteo lucca proposed dark energy-dark matter interactions as a possible solution to the S8 tension
Matteo Lucca investigated dark energy-dark matter interaction as a possible solution to the S8 tension. In particular, in order to address S8 tension, he proposed a scenario in which dark energy is a dynamical fluid, whose energy density can be transferred to the dark matter via a coupling function proportional to the energy density of the dark energy. Their study recently appeared in Arxiv.
Not just dark energy-dark matter interaction, several researchers also considered dark matter-dark energy interaction scenario. But, the cosmological features of both interactions are diametrically differ from each other depending on whether the energy density is flowing from the DE to the DM or vice versa.
In dark energy-dark matter interaction (iDEDM), because of the additional energy injected from the DE into the DM over the cosmic history, there has an overall suppression of the DM energy density with respect to the ΛCDM scenario. As a consequence, this results on the one hand in a decreased value of the Hubble parameter at late times, worsening the Hubble tension, and on the other hand in a delay of the radiation-matter equality from which follows a suppression of the matter power spectrum, which can alleviate the S8 tension. The opposite is true for the dark matter-dark energy interaction (iDMDE) case.
For these reasons, great attention has been dedicated to the iDMDE model in the context of the Hubble tension, although it has been explicitly shown by Lucca and Hooper in their previous study that, also this particular interacting DE scenario is unable to address the Hubble tension due to the no-go theorem that arises when Baryon Acoustic Oscillation (BAO) and SNIa data are accounted for. However, surprisingly, to his knowledge the iDEDM scenario has never been thoroughly investigated as a possible solution to the S8 tension. Thus, this inspired him to undertake this task in this study.
He found that, against data from Planck, BAO and Pantheon, the model
can significantly reduce the significance of the tension,
does so without exacerbating nor introducing any other tension (such as the H0 tension) and,
without worsening the fit to the considered data sets with respect to the ΛCDM model.
He also tested the model against data from weak lensing surveys such as KiDS and DES, and found that the model’s ability to address the S8 tension further improves, without a significant impact on any other parameter nor statistical measure.
A five-year quest to map the Universe and unravel the mysteries of ‘dark energy’ began officially on May 17 at Kitt Peak National Observatory near Tucson, Arizona. To complete its quest, the Dark Energy Spectroscopic Instrument (DESI) will capture and study the light from tens of millions of galaxies and other distant objects in the Universe.
DESI is an international science collaboration managed by the Lawrence Berkeley National Laboratory (Berkeley Lab) with primary funding from the U.S. Department of Energy (DOE) Office of Science. The Chinese Participant Group (CPG) organized by National Astronomical Observatories, Chinese Academy of Science (NAOC) has joined in the DESI collaboration through the imaging contribution of the Beijing-Arizona Sky Survey (BASS).
ZOU Hu, who is the duty-PI of BASS and NAOC representative in DESI, said, “The BASS team at NAOC has spent about five years in collecting the astronomical images in the northern sky, which paves the way for the DESI spectroscopic survey. Now the DESI sets sail to explore the universe.” ZOU added, “The DESI is a really spectra factory that can collect 5,000 spectra every 20 minutes.”
By gathering light from some 30 million galaxies, project scientists say DESI will help them construct a 3D map of the universe with unprecedented detail. The data will help them better understand the repulsive force associated with “dark energy” that drives the acceleration of the expansion of the universe across vast cosmic distances.
What sets DESI apart from previous sky surveys? The project director, Berkeley Lab’s Michael Levi, said, “We will measure10 times more galaxy spectra than ever obtained. These spectra get us a third dimension.” Instead of two-dimensional images of galaxies, quasars, and other distant objects, he explained, the instrument collects light, or spectra, from the cosmos such that it “becomes a time machine where we place those objects on a timeline that reaches as far back as 11 billion years ago.”
“It’s been a long journey from the first steps that we took almost a decade ago to design the survey then to decide which targets to observe, and now to have the instruments so that we can achieve the science goals,” said project co-spokesperson Nathalie Palanque-Delabrouille. She added “It’s very exciting to see where we stand today.”
The group leader of the Sky Survey and TMT Development at NAOC, XUE Suijian, said, “Scores of researchers around the world have tested thousands of DESI’s component parts and validated the survey.” He added, “After completing the imaging surveys, NAOC members have been involved in the DESI commissioning and survey validations during the past two years.”
The formal start of DESI’s five-year survey follows a four-month trial run of its custom instrumentation that captured spectra from four million galaxies -more than the combined output of all previous spectroscopic surveys.
The DESI instrument includes 5,000 robotically controlled optical fibers to gather spectroscopic data from an equal number of objects. Spectra collected by DESI are the components of light corresponding to the colors of the rainbow. Their characteristics, including wavelength, reveal information such as the chemical composition of objects being observed as well as information about their relative distance and velocity.
As the Universe expands, galaxies move away from each other, and their light is shifted to longer, redder wavelengths. The more distant the galaxy, the greater its “redshift.” By measuring galaxy redshifts, DESI researchers will create a 3D map of the Universe. The detailed distribution of galaxies in the map is expected to yield new insights on the influence and nature of dark energy.
NAOC astronomers will further participate in the DESI observing, cooperate with the scientists from the DESI collaboration, and utilize the DESI spectra to understand the special types of stars and galaxies, galaxy evolution, and cosmology, etc.
Featured image: The disk of the Andromeda Galaxy (M31) is targeted by a single DESI pointing, represented by the large, pale green, circular overlay. The smaller circles within this overlay represent the regions accessible to each of the 5,000 DESI robotic fiber positioners. (Image by DESI collaboration and DESI Legacy Imaging Surveys)
Garoffolo and colleagues built an estimator, by combining luminosity distance measurements from Gravitational waves (GW) and Supernova sources (SN) , for the direct detection of the signal of dark energy fluctuations, that does not rely on non-gravitational interactions between dark energy (DE) and known particles.
They mentioned that this signal can not be mimicked by other effects and would provide convincing evidence for the existence of the DE field or modifications of the laws of gravity.
In addition, in order to detect this signal, very precise measurements of SN/GW events are required. Alongwith, the higher number of events, to deal with possible systematic effects.
Finally, they demonstrated that this signal would be directly detected by future SN surveys and space-based interferometers, if one decreases the statistical error on each measure.
Over the last decades, a variety of cosmological data have confirmed ΛCDM as the standard model of cosmology. It provides understanding of the Big Bang cosmology, inflation, the matter-antimatter asymmetry in the universe, the nature of dark energy, etc. Despite its successes, the physical nature of its main components still eludes us. In particular, understanding whether cosmic acceleration is sourced by a cosmological constant, Λ, or rather by dynamical dark energy (DE) or modifications of the laws of gravity (MG) is one of the main science drivers of upcoming cosmological missions. In the presence of DE/MG, the dynamical degrees of freedom of the theory change, generally with the appearance of a new scalar field to which we broadly refer as the “DE field”. The latter, leaves imprints not only on the dynamics of the Universe, but also on the clustering and growth of large-scale cosmological structures.
The detection of gravitational waves (GW) has opened a new observational window onto our Universe, promising to offer complementary probes to shed light on cosmic expansion. GW events at cosmological distances can be used as “standard sirens” (a nod to “standard candles”) for measuring the expansion rate of the universe. This recent approach is complementary to measuring the luminosity distance of standard candles, like Type-Ia supernovae (SN).
On the homogeneous and isotropic background, luminosity distances depend only on redshift, leading to the standard distance-redshift relation. Inhomogeneities in the Universe induce a dependence of the distances on direction. Fluctuations in the EM luminosity distance constitute an important probe for cosmology.
Previous papers showed that, in presence of DE/MG, the gravitational wave (GW) luminosity distance generally differs from the one traced by electromagnetic (EM) signals, both at the unperturbed, background level and in its large-scale fluctuations. Importantly, fluctuations in the EM luminosity distance are affected by the DE field only indirectly while, linearized fluctuations of the GW luminosity distance contain contributions directly proportional to the clustering of the DE field.
Now, considering this, Garoffolo and colleagues built an estimator, by combining luminosity distance measurements from GW and SN sources, for the direct detection of the signal of DE clustering (or imprint of the DE fluctuations), that does not rely on non-gravitational interactions between DE and known particles. They mentioned that this signal can not be mimicked by other effects and would provide convincing evidence for the existence of the DE field or modifications of the laws of gravity.
“If DE does not directly couple to known particles through nongravitational interactions, ours is a promising method to pursue its direct detection.” toldAlice Garoffolo, first author of the study, “Even if the DE clustering signal is below cosmic variance, any detection of our joint estimator would be a convincing proof of a running Planck mass.”
In addition, their proposed approach allows to probe the DE field at cosmological scales, far from sources that can hide its presence by means of screening mechanisms. (Note: Screening mechanisms were first categorized by Austin Joyce and colleagues into three broad classes: mechanisms which become active in regions of high Newtonian potential, those in which first derivatives become important, and those for which second derivatives are important. For more, please refer their paper.)
But, on the other hand, it has been suggested that, one should leverage as much as possible on the precision of the measurement; for instance, given the number of SN/GW events (of order 106, at least in the higher redshift bins) that can be observed with future SN surveys and space-based interferometers, a detection would be possible, if one decreases the statistical error on each measure, according to table I below.
Finally, they considered an ideal case for their estimates: the number of events needed for a detection might be higher to deal with possible systematic effects. This suggested that future facilities might have to develop new technologies and observational strategies to meet these detection goals.
“We leave it to future work to determine whether a detection of the signal we propose can be aided by studying additional MG models, synergies with large scale structure surveys or considering different sources of GW/EM signals.”
— concluded authors of the study
Reference: Alice Garoffolo, Marco Raveri, Alessandra Silvestri, Gianmassimo Tasinato, Carmelita Carbone, Daniele Bertacca, and Sabino Matarrese, “Detecting dark energy fluctuations with gravitational waves”, Phys. Rev. D 103, 083506 – Published 12 April 2021. https://doi.org/10.1103/PhysRevD.103.083506Link to paper
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The universe we see is only the very tip of the vast cosmic iceberg.
The hundreds of billions of galaxies it contains, each of them home to billions of stars, planets and moons as well as massive star-and-planet-forming clouds of gas and dust, and all of the visible light and other energy we can detect in the form of electromagnetic radiation, such as radio waves, gamma rays and X-rays — in short, everything we’ve ever seen with our telescopes — only amounts to about 5% of all the mass and energy in the universe.
Along with this so-called normal matter there is also dark matter, which can’t be seen, but can be observed by its gravitational effect on normal, visible matter, and makes up another 27% of the universe. Add them together, and they only total 32% of the mass of the universe — so where’s the other 68%?
So what exactly is dark energy? Put simply, it’s a mysterious force that’s pushing the universe outward and causing it to expand faster as it ages, engaged in a cosmic tug-of-war with dark matter, which is trying to pull the universe together. Beyond that, we don’t yet understand what dark energy is, but Penn State astronomers are at the core of a group that’s aiming to find out through a unique and ambitious project 16 years in the making: HETDEX, the Hobby-Eberly Telescope Dark Energy Experiment.
“HETDEX has the potential to change the game,” said Associate Professor of Astronomy and Astrophysics Donghui Jeong.
Dark energy and the expanding universe
Today there is consensus among astronomers that the universe we inhabit is expanding, and that its expansion is accelerating, but the idea of an expanding universe is less than a century old, and the notion of dark energy (or anything else) accelerating that expansion has only been around for a little more than 20 years.
In 1917 when Albert Einstein applied his general theory of relativity to describe the universe as a whole, laying the foundations for the big bang theory, he and other leading scientists at that time conceived of the cosmos as static and nonexpanding. But in order to keep that universe from collapsing under the attractive force of gravity, he needed to introduce a repulsive force to counteract it: the cosmological constant.
It wasn’t until 1929 when Edwin Hubble discovered that the universe is in fact expanding, and that galaxies farther from Earth are moving away faster than those that are closer, that the model of a static universe was finally abandoned. Even Einstein was quick to modify his theories, by the early 1930s publishing two new and distinct models of the expanding universe, both of them without the cosmological constant.
But although astronomers had finally come to understand that the universe was expanding, and had more or less abandoned the concept of the cosmological constant, they also presumed that the universe was dominated by matter and that gravity would eventually cause its expansion to slow; the universe would either continue to expand forever, but ever-increasingly slowly, or it would at some point cease its expansion and then collapse, ending in a “big crunch.”
“That’s the way we thought the universe worked, up until 1998,” said Professor of Astronomy and Astrophysics Robin Ciardullo, a founding member of HETDEX.
That year, two independent teams — one led by Saul Perlmutter at Lawrence Berkeley National Laboratory, and the other led by Brian Schmidt of the Australian National University and Adam Riess of the Space Telescope Science Institute — would nearly simultaneously publish astounding results showing that the expansion of the universe was in fact accelerating, driven by some mysterious antigravity force. Later that year, cosmologist Michael Turner of the University of Chicago and Fermilab coined the term “dark energy” to describe this mysterious force.
The discovery would be named Science magazine’s “Breakthrough of the Year” for 1998, and in 2011 Perlmutter, Schmidt and Reiss would be awarded the Nobel Prize in physics.
More than 20 years after the discovery of dark energy, astronomers still don’t know what, exactly, it is.
“Whenever astronomers say ‘dark,’ that means we don’t have any clue about it,” Jeong said with a wry grin. “Dark energy is just another way of saying that we don’t know what’s causing this accelerating expansion.”
There are, however, a number of theories that attempt to explain dark energy, and a few major contenders.
Perhaps the most favored explanation is the previously abandoned cosmological constant, which modern-day physicists describe as vacuum energy. “The vacuum in physics is not a state of nothing,” Jeong explained. “It is a place where particles and antiparticles are continuously created and destroyed.” The energy produced in this perpetual cycle could exert an outward-pushing force on space itself, causing its expansion, initiated in the big bang, to accelerate.
Unfortunately, the theoretical calculations of vacuum energy don’t match the observations — by a factor of as much as 10120, or a one followed by 120 zeroes. “That’s very, very unusual,” Jeong said, “but that’s where we’ll be if dark energy turns out to be constant.” Clearly this discrepancy is a major issue, and it could necessitate a reworking of current theory, but the cosmological constant in the form of vacuum energy is nonetheless the leading candidate so far.
As a result of its design, HETDEX is collecting a massive amount of data, extending well beyond its intended targets and providing additional insights into things like dark matter and black holes, the formation and evolution of stars and galaxies, and the physics of high-energy cosmic particles such as neutrinos.
Another possible explanation is a new, yet-undiscovered particle or field that would permeate all of space; but so far, there’s no evidence to support this.
A third possibility is that Einstein’s theory of gravity is incorrect. “If you start from the wrong equation,” Jeong said, “then you get the wrong answer.” There are alternatives to general relativity, but each has its own issues and none has yet displaced it as the reigning theory. For now, it’s still the best description of gravity we’ve got.
Ultimately, what’s needed is more and better observational data — precisely what HETDEX was designed to collect like no other survey has done before.
A map of stars and sound
“HETDEX is very ambitious,” Ciardullo said. “It’s going to observe a million galaxies to map out the structure of the universe going over two-thirds of the way back to the beginning of time. We’re the only ones going out that far to see the dark energy component of the universe and how it’s evolving.”
Ciardullo, an observational astronomer who studies everything from nearby stars to faraway galaxies and dark matter, is HETDEX’s observations manager. He’s quick to note, though, that he’s got help in that role (from Jeong and others) and that he and everyone else on the project wears more than one hat. “This is a very big project,” he said. “It’s over $40 million. But if you count heads, it’s not very many people. And so we all do more than one thing.”
Jeong, a theoretical astrophysicist and cosmologist who also studies gravitational waves, was instrumental in laying the groundwork for the study and is heavily involved in the project’s data analysis — and he’s also helping Ciardullo determine where to point the 10-meter Hobby-Eberly Telescope, the world’s third largest. “It’s kind of interesting,” he noted with a chuckle, “a theorist telling observers where to look.”
“We’re the only ones going out that far to see the dark energy component of the universe and how it’s evolving.”
—Robin Ciardullo, Penn State professor of astronomy and astrophysics
While other studies measure the universe’s expansion using distant supernovae or a phenomenon known as gravitational lensing, where light is bent by the gravity of massive objects such as galaxies and black holes, HETDEX is focused on sound waves from the big bang, called baryonic acoustic oscillations. Although we can’t actually hear sounds in the vacuum of space, astronomers can see the effect of these primordial sound waves in the distribution of matter throughout the universe.
During the first 400,000-or-so years following the big bang, the universe existed as dense, hot plasma — a particle soup of matter and energy. Tiny disturbances called quantum fluctuations in that plasma set off sound waves, like ripples from a pebble tossed into a pond, which helped matter begin to clump together and form the universe’s initial structure. The result of this clumping is evident in the cosmic microwave background (also called the “afterglow” of the big bang), which is the first light, and the farthest back, that we can see in the universe. And it’s also imprinted in the distribution of galaxies throughout the universe’s history — like the ripples on our pond, frozen into space.
“The physics of sound waves is pretty well known,” Ciardullo said. “You see how far these things have gone, you know how fast the sound waves have traveled, so you know the distance. You have a standard ruler on the universe, throughout cosmic history.”
As the universe has expanded so has the ruler, and those variances in the ruler will show how the universe’s rate of expansion, driven by dark energy, has changed over time.
“Basically,” Jeong said, “we make a three-dimensional map of galaxies and then measure it.”
New discovery space
To make their million-galaxy map, the HETDEX team needed a powerful new instrument.
A set of more than 150 spectrographs called VIRUS (Visible Integral-Field Replicable Unit Spectrographs), mounted on the Hobby-Eberly Telescope, gathers the light from those galaxies into an array of some 35,000 optical fibers and then splits it into its component wavelengths in an ordered continuum known as a spectrum.
Galaxies’ spectra reveal, among other things, the speed at which they are moving away from us — a measurement known as “redshift.” Due to the Doppler effect, the wavelength of an object moving away from its observer is stretched (think of a siren that gets lower in pitch as it speeds away), and an object moving toward its observer has its wavelength compressed, like that same siren increasing in pitch as it gets nearer. In the case of receding galaxies, their light is stretched and thus shifted toward the red end of the spectrum.
Measuring this redshift allows the HETDEX team to calculate the distance to those galaxies and produce a precise three-dimensional map of their positions.
Among the galaxies HETDEX is observing are what are known as Lyman-alpha galaxies — young star-forming galaxies that emit strong spectral lines at specific ultraviolet wavelengths.
“We’re using Lyman-alpha-emitting galaxies as a ‘tracer particle,’” explained Research Professor of Astronomy and Astrophysics Caryl Gronwall, who is also a founding member of HETDEX. “They’re easy to find because they have a very strong emission line, which is easy to find spectroscopically with the VIRUS instrument. So we have this method that efficiently picks out galaxies at a fairly high redshift, and then we can measure where they are, measure their properties.”
Gronwall, who along with Ciardullo has been studying Lyman-alpha galaxies for nearly 20 years, leads HETDEX’s efforts in this area, while Associate Professor of Astronomy and Astrophysics Derek Fox lends his expertise to calibrating the VIRUS instrument, using incidental observations of stars with well-known properties to fine-tune its spectra.
“Every shot we take with HETDEX, we observe some stars on the fibers,” Fox explained. “That’s an opportunity, because the stars are telling you how sensitive your experiment is. If you know the brightness of the stars and you see the data that you collect on them, it offers an opportunity to keep your calibration on point.”
“HETDEX has the potential to change the game.”
—Donghui Jeong, Penn State associate professor of astronomy and astrophysics
One of HETDEX’s biggest strengths is that it was designed as a blind survey — observing broad swaths of sky instead of specific, predetermined objects. “Nobody has tried doing a survey like this before,” Ciardullo said. “It’s always ‘Find your objects, then do the spectroscopy.’ We’re the first ones to try to do a whole lot of spectroscopy and then figure out what we saw.”
As a result of this design, HETDEX is collecting a massive amount of data, extending well beyond its intended targets and providing additional insights into things like dark matter and black holes, the formation and evolution of stars and galaxies, and the physics of high-energy cosmic particles such as neutrinos.
“That’s very different and very interesting,” Jeong said. “We have huge discovery space.”
Ciardullo added, “One thing you can infer — if you first have to see an object before pointing your spectroscope there, well that’s fine, but it requires that the object be able to be seen. HETDEX can observe spectra of things that you can’t see.”
This means that in addition to the known data it’s collecting, HETDEX is opening a window to unexpected findings, discoveries yet unforeseen. “We will be a pathfinder for more experiments,” Ciardullo said, and that sentiment is echoed by others on the team, including Fox.
“We’re definitely going to be blazing trails out there,” he said. “There’s big, big potential for really exciting discoveries.”
Back to roots, and beyond
The futuristic science of HETDEX is, in a strange twist, very much in line with the ideas that drove the development of the Hobby-Eberly Telescope (HET) nearly 40 years ago.
“HET was initially conceived as the Penn State Spectroscopic Survey Telescope,” explained Professor Emeritus of Astronomy and Astrophysics Larry Ramsey, who invented the telescope in 1983 with then Penn State colleague Dan Weedman, and later served as chairman of the HET’s board of directors. “The original mission was to conduct spectroscopic surveys, and in the almost 20 years between when we first dedicated the telescope and when we started HETDEX, the telescope was not really doing surveys. So in a very real sense HETDEX is taking the HET back to its roots, and it has grown into a really interesting project.”
“The scale of this survey is very futuristic, even now,” Jeong said. Recalling a recent cosmology conference, he related a discussion about the future of galactic surveys. “I sat there and listened, and it was basically what we’re doing,” he said. “HETDEX is a future survey that exists now.”
In addition to what HETDEX discovers about dark energy, the data it’s collecting will also provide fodder for future studies far beyond the scope of its own mission. And chances are, HETDEX will continue doing “spacebreaking” science on the distant, high-redshift universe for quite a few years to come.
“Even currently planned future surveys don’t go beyond HETDEX,” Jeong said. “I think we will still be at the forefront, even 10 years from now.”
Featured image: The Hobby-Eberly Telescope. Credit: Marty Harris, McDonald Observatory, UT Austin
The extended Baryon Oscillation Spectroscopic Survey (eBOSS) collaboration has released its latest scientific results. These results include two studies on dark energy led by Prof. ZHAO Gongbo and Prof. WANG Yuting, respectively, from National Astronomical Observatories of the Chinese Academy of Sciences(NAOC).
Based on eBOSS observations, Prof. ZHAO’s team measured the history of cosmic expansion and structure growth in a huge volume of the past universe, corresponding to a distance range between 0.7 and 1.8 billion light years away from us. This volume had never been probed before.
This study took advantage of a method called “multi-tracer analysis,” which mitigated the observational systematics proposed and implemented by Prof. ZHAO and Prof. WANG.
“This work detected the existence of dark energy at a significance of 11 sigma, which is the strongest evidence ever on dark energy from galaxy surveys,” said Prof. ZHAO. “The eBOSS observations are consistent with the dynamical dark energy probed by our team using the BOSS survey four years ago.”
One of the biggest challenges of the cosmological implications of galaxy surveys is data analysis, especially developing new methods to improve statistical accuracy and mitigate systematics. eBOSS, as the first galaxy survey that observes multiple types of galaxies in a vast cosmic volume, makes it possible to use multi-tracer analysis.
“Cross-correlating multiple types of galaxies is an efficient way to reduce statistical uncertainties, with observational systematics mitigated at the same time, which is key to obtaining robust cosmological results,” said Prof. WANG.
Dark energy dominates the current universe, so it is vitally important to reveal its nature. The eBOSS collaboration, which consists of over 30 top research institutes on astronomy around the world including NAOC, was formed to tackle the problem of dark energy. Operating since 2014, eBOSS has taken over 1 million spectra in the redshift range of 0.6.
Although eBOSS has completed its mission, it is a starting point for the next chapter. The experience with eBOSS is valuable for cosmology involving larger galaxy surveys, including the Chinese Space Station Telescope (CSST), Dark Energy Spectroscopic Instrument (DESI) and Prime Focus Spectrograph (PFS).
This work is supported by the Natural Science Foundation of China, Ministry of Science and Technology of China, and Chinese Academy of Sciences.
Reference: Gong-Bo Zhao, Yuting Wang, Atsushi Taruya, Weibing Zhang, Héctor Gil-Marín, Arnaud de Mattia, Ashley J Ross, Anand Raichoor, Cheng Zhao, Will J Percival, Shadab Alam, Julian E Bautista, Etienne Burtin, Chia-Hsun Chuang, Kyle S Dawson, Jiamin Hou, Jean-Paul Kneib, Kazuya Koyama, Hélion du Mas des Bourboux, Eva-Maria Mueller, Jeffrey A Newman, John A Peacock, Graziano Rossi, Vanina Ruhlmann-Kleider, Donald P Schneider, Arman Shafieloo, The completed SDSS-IV extended Baryon Oscillation Spectroscopic Survey: a multitracer analysis in Fourier space for measuring the cosmic structure growth and expansion rate, Monthly Notices of the Royal Astronomical Society, Volume 504, Issue 1, June 2021, Pages 33–52, https://doi.org/10.1093/mnras/stab849
For the first time, DES scientists can combine measurements of the distribution of matter, galaxies, and galaxy clusters to advance our understanding of dark energy.BY NATHAN COLLINS
The universe is expanding at an ever-increasing rate, and while no one is sure why, researchers with the Dark Energy Survey (DES) at least had a strategy for figuring it out: They would combine measurements of the distribution of matter, galaxies and galaxy clusters to better understand what’s going on.
Reaching that goal turned out to be pretty tricky, but now a team led by researchers at the Department of Energy’s SLAC National Accelerator Laboratory, Stanford University and the University of Arizona have come up with a solution. Their analysis, published today in Physical Review Letters, yields more precise estimates of the average density of matter as well as its propensity to clump together – two key parameters that help physicists probe the nature of dark matter and dark energy, the mysterious substances that make up the vast majority of the universe.
When DES set out in 2013 to map an eighth of the sky, the goal was to gather four kinds of data: the distances to certain types of supernovae, or exploding stars; the distribution of matter in the universe; the distribution of galaxies; and the distribution of galaxy clusters. Each tells researchers something about how the universe has evolved over time.
Ideally, scientists would put all four data sources together to improve their estimates, but there’s a snag: The distributions of matter, galaxies, and galaxy clusters are all closely related. If researchers don’t take these relationships into account, they will end up “double counting,” placing too much weight on some data and not enough on others, To says.
To avoid mishandling all this information, To, University of Arizona astrophysicist Elisabeth Krause and colleagues have developed a new model that could properly account for the connections in the distributions of all three quantities: matter, galaxies, and galaxy clusters. In doing so, they were able to produce the first-ever analysis to properly combine all these disparate data sets in order to learn about dark matter and dark energy.
Adding that model into the DES analysis has two effects, To says. First, measurements of the distributions of matter, galaxies and galaxy clusters tend to introduce different kinds of errors. Combining all three measurements makes it easier to identify any such errors, making the analysis more robust. Second, the three measurements differ in how sensitive they are to the average density of matter and its clumpiness. As a result, combining all three can improve the precision with which the DES can measure dark matter and dark energy.
In the new paper, To, Krause and colleagues applied their new methods to the first year of DES data and sharpened the precision of previous estimates for matter’s density and clumpiness.
Now that the team can incorporate matter, galaxies and galaxy clusters simultaneously in their analysis, adding in supernova data will be relatively straightforward, since that kind of data is not as closely related with the other three, To says.
“The immediate next step,” he says, “is to apply the machinery to DES Year 3 data, which has three times larger coverage of the sky.” This is not as simple as it sounds: While the basic idea is the same, the new data will require additional efforts to improve the model to keep up with the higher quality of the newer data, To says.
“This analysis is really exciting,” Wechsler said. “I expect it to set a new standard in the way we are able to analyze data and learn about dark energy from large surveys, not only for DES but also looking forward to the incredible data that we will get from the Vera Rubin Observatory’s Legacy Survey of Space and Time in a few years.”
The research was a collaborative effort within the Dark Energy Survey and was supported by the National Science Foundation and the Department of Energy’s Office of Science.
Featured image: A map of the sky showing the density of galaxy clusters, galaxies and matter in the universe over the part of the sky observed by the Dark Energy Survey. The left panel shows the galaxy density in that part of the sky, while the middle panel shows matter density and the right shows galaxy cluster density. Red areas are more dense, and blue areas are less dense, than average. (Chun-Hao To/Stanford University, SLAC)
Citation: C. To et al. (DES Collaboration), “Dark Energy Survey Year 1 Results: Cosmological Constraints from Cluster Abundances, Weak Lensing, and Galaxy Correlations”, Physical Review Letters, 6 April 2021 (10.1103/PhysRevLett.126.141301)