Tag Archives: #cosmology

The Mystery Of The Missing Dark Matter (Cosmology)

New distance measurements of the diffuse spheroid galaxy Ngc 1052-Df2 place this galaxy at a distance of 72 million light years and confirm that the galaxy is practically devoid of dark matter, a very rare case in the galaxy landscape. This absence of dark matter compared to other galaxies suggests that dark matter exists as a real physical entity and not as a result of a different law of gravitation on a galactic scale.

According to the current paradigm, dark matter makes up about 86% of all matter in our Universe. Its peculiarity is that it does not interact electromagnetically like ordinary matter, but only by gravitational way . For this reason it is difficult to study it, in fact it can only be detected on a large scale by observing the gravitational effects it causes on ordinary matter: unfortunately there is no experimental detection.of dark matter particles. The presence of this matter has been deduced thanks to studies on the velocity curves of spiral galaxies: as we move away from the nucleus of a galaxy, the stars do not decrease their speed as one might expect, but continue to move. quickly. If Newton’s law of gravity holds, this excess of speed indicates that most of the mass of galaxies is made up of invisible matter capable of holding the stars of which they are composed bound together with its own force of gravity: unlike galaxies. they would fall apart. Dark matter in the evolution of the Universe is very important because it is thanks to its intense gravitational effects that, within immense haloes of dark mattergalaxies were formed . Otherwise, after the Big Bang, ordinary matter would never have undergone any process of gravitational collapse and galaxies would not have formed. From this theoretical framework it is expected that each galaxy contains a consistent amount of dark matter: for example the value of the average ratio between dark matter and ordinary , measured for galaxies such as our Milky Way, is of the order of 30 times and increases both for more massive galaxies, and for less massive galaxies.

However, things seem more complex than that, at least as far as the galaxy Ngc 1052-Df2 is concerned . It is an ultra-diffuse galaxy with low surface brightness that is prospectively located in the constellation of the Whale, identified thanks to a large-field survey of the group of galaxies of Ngc 1052. The galaxy contains so little ordinary matter that it is practically transparent, so much is it It is true that in the images that portray it you can see the background galaxies much further away. Morphologically, this galaxy has a spheroidal appearance and does not appear to have a core, spiral arms or a disk of stars. The geometric dimensions are similar to those of the Milky Way.

In a March 2018 article published in Nature, the results of the radial velocity measurements of 10 luminous globular clusters belonging to this evanescent galaxy were published for the estimation of the total mass of the system. The result was that the ratio of dark to bright matter in Ngc 1052-Df2 was about 1, a value about 400 times lower than expected and in stark contrast to what is observed in other galaxies. Put simply, the case of NGC1052-DF2 showed that dark matter is not always coupled with baryon matter , at least on a galactic scale. To confirm this incredible result, the discovery team, led by Pieter van Dokkum of Yale University, focused on precise distance measurementby Ngc 1052-Df2, publishing a new paper in The Astrophysical Journal Letters . In the work of 2018, the distance of the galaxy was assumed to be similar to that of the group of galaxies to which it seemed to belong, namely that of Ngc 1052 at about 65 million light years from us. How does distance fit into estimating the relationship between dark and ordinary matter? To understand this, just think of the fact that the estimation of the mass of a star can be done by measuring its intrinsic brightness and this is obtained by measuring both the apparent brightness and the distance at which the star is located. By scaling this reasoning on a galactic scale we understand that if Df2 were closer to Earth than the 65 million light years adopted, thenits stars would be intrinsically weaker and less massive , so the luminous matter would make a minor contribution to the total mass (which is measured with the radial velocity of globular clusters) and the ratio between dark and luminous matter would increase accordingly. Distance measurement thus becomes a crucial parameter for determining the amount of luminous matter in the galaxy.

To measure the distance of a galaxy you need ” standard candles “, ie stars whose intrinsic brightness is known a priori . The team of astronomers, using the “Hubble” space telescope, focused on measuring the apparent brightness of the red giants located on the periphery of Ngc 1052-Df2 and which, during their evolution, all reach the same brightness peak. In this way, the difference between intrinsic and apparent brightness can be used to measure large intergalactic distances. The new distance estimate tells us that Df2 is 72 million light years awaythat is, the galaxy is further away than the original estimate of 65 million light years. From here it follows that Df2 is really devoid of dark matter, it is not an observational bias .

Moreover, Df2 is not the only galaxy without dark matter, another galaxy, Ngc 1052-Df4 , is also devoid of dark matter. In this case, however, some scientists suggest that dark matter may have been removed from the galaxy due to tidal forces exerted by another passing galaxy.

The discovery of these galaxies devoid of dark matter, paradoxically, confirms that dark matter really exists. In fact, if dark matter were only an effect of a gravitational law different from the Newtonian one, all galaxies should show its presence. The fact that there are galaxies without dark matter means that something is really missing in their structure. Understanding why Df2 is devoid of dark matter will require further observation, the mystery continues.

Featured image: The galaxy poor in dark matter Ngc 1052-Df2 taken with the Hubble Advanced Camera for Surveys between December 2020 and March 2021. The galaxy is so poor in matter that, through it, you can see the background galaxies (Credits: Nasa , Esa, STScI, Zili Shen (Yale), Pieter van Dokkum (Yale), Shany Danieli (Ias), Alyssa Pagan (STScI))

To know more:

Provided by INAF

A Look At The Galactic Plane With Askap (Cosmology)

Leveraging the Australian forerunner of the Ska project, the first map of a section of the Milky Way’s galactic plane (about 40 square degrees wide) was created with an angle of sensitivity and resolution never before achieved by an infrastructure in the southern hemisphere. The results were published in Mnras by a group of radio astronomers led by INAF and the University of Macquarie in Sydney.

A group of radio astronomers, led by the National Institute of Astrophysics (INAF) and the University of Macquarie in Sydney, made the first radio observations of a large section of the galactic plane of the Milky Way with the Australian Ska Pathfinder (Askap) , developed and managed by the Commonwealth Scientific and Industrial Research Organization (Csiro). Specifically, the region mapped by the researchers includes the entire area of ​​the Scorpio survey (Stellar Continuum Originating from Radio Physics In Ourgalaxy), one of the numerous exploration projects of the broader Evolutionary Map of the Universe (Emu) program, which consists of observation of the whole southern hemisphere with Askap, one of theprecursors of the Ska project . The observations, reported in two articles published in the Monthly Notices of the Royal Astronomical Society , were made in 2018 with the interferometer not yet fully deployed (15 of the 36 antennas were operational at the time), covering a total area of ​​about 40 degrees squares. 

As part of the preparatory activities for the EMU survey , Askap’s antennas were pointed towards the tail of the constellation of Scorpio. The so-called Scorpio field was included among the first targetsscientific studies of Askap, thanks to the preliminary work carried out by the Italian team of INAF using the Australia Telescope Compact Array (Atca), which allowed to achieve a series of important scientific results and to develop skills in the reduction and analysis of radio data of the plan galactic. More than 3,600 compact radio sources have been extracted from the Scorpio field, many of which are unclassified. All the sources previously classified as HII regions, areas rich in ionized hydrogen associated with star formation sites, or as planetary nebulae, the last evolutionary phases of stars of intermediate mass, have been revealed and new ones have been discovered, significantly increasing the number of objects belonging to these different galactic populations.radio quiet and to discover numerous extended sources, not classified and belonging to the class of so-called “galactic bubbles”, which constitute a new sample within which to identify new supernova remains. 

Composite image of a portion of the Scorpio field. In green the infrared data collected by Spitzer / Glimpse, in red those collected by Herschel / Hi-Gal and in blue the radio data collected by Askap. By superimposing infrared data (which track dust) on radio maps (which track either ionized gas or synchrotron), radio and infrared coincide in star-forming regions, while in the case of supernova remnants (Snr) only radium is visible . Credits: G. Umana / Inaf

“Scorpio is the only galactic field observed so far with Askap and is therefore particularly important for the characterization of some galactic populations”, explains Grazia Umana , principal investigator of the survey and first author of one of the two articles, as well as a researcher at INAF of Catania, «because it provides a solid base level from which to start to better design some aspects of the EMU survey . In addition to the discovery of numerous galactic radio sources, these observations have highlighted Askap’s unique feature of mapping complex objects at various angular scales, an extremely useful feature especially in the case of studies of the galactic plane. This is the result of a skilful design byarray that is sensitive to both compact objects and extended and diffuse emission. On the basis of these first results of observations of the galactic plane with Askap we can have only a small taste of the potential of the Ska project in the field of galactic radio astronomy ». 

The galactic plane is the place in the Milky Way where the solar system resides: it contains countless stars, dust and gas clouds, as well as a significant amount of dark matter. Studying the plan of the Milky Way has always been one of the most important objectives of radio astronomers, but the presence of diffuse emission in the galaxy makes it difficult to obtain artifact-free images: this effectively reduces the quality of the final images making data analysis a task. particularly challenging. Many of these problems have been mitigated using different approaches and increasingly complex algorithms, but due to the large amount of data provided by tools such as Askap, human intervention at each stage of data reduction is not possible and this requires a different approach. .

” Numerous difficulties have arisen in the data reduction phase”, underlines Simone Riggi , researcher at INAF in Catania and first author of the survey catalog article , “because the standard techniques are currently optimized for extragalactic fields in which the emission diffuse that permeates the galactic plane is not present. It was therefore necessary to develop new procedures for data calibration and define new strategies in the data acquisition phase. What we have learned will allow us to contribute to the design of the EMU survey , optimizing its scientific return also for galactic science.An important part of the work done with the Ska precursors is to gain experience in managing the representative data of the Ska project. A significant challenge with these huge datasets, beyond data reduction itself, is to automatically find and classify radio sources. ‘ 

The Milky Way extending over the Askap radio telescope operated by Csiro (the Australian Scientific Agency) at the Murchison Radio-astronomy Observatory in Western Australia. Askap is one of the forerunners of the Ska project. Credits: Csiro / A. Cherney

The Scorpio field was used as a test bench to test the Caesar source extraction tool developed by the Italian team on real data and in the presence of diffuse emission . A first catalog of compact radio sources and their components was produced, which will be subsequently updated when the new Askap observations of the Scorpio field with the complete array are available, scheduled for the end of 2021. “Multi-frequency data and the expected increase in sensitivity and spatial resolution will allow us to measure the spectral index for all sources, enabling further progress in our classification studies, ”adds Riggi.

“The data collected at that early stage of Askap also demonstrates its excellent sensitivity to extended radio emissions,” says Andrew Hopkins , head of the EMU project for Macquarie University. «A fundamental result to allow us to detect these important structures in the Milky Way, allowing us to deepen our knowledge on the formation and evolution of stars in the galaxy».

The EMU project will also extend to part of the northern hemisphere, covering 75 percent of the sky observed at the 1.4 GHz frequency, with better angular resolution and sensitivity than achieved so far. The researchers will observe a large fraction of the galactic plane and will be able to produce a wide-field atlas of the Milky Way’s continuous radio emission, with unprecedented results in terms of sensitivity and angular resolution, which will have a major impact in star formation studies. of the galactic structure and stellar evolution.

«New Askap observations of the galactic plane as part of the EMU survey and subsequently with the Ska project will allow us to explore a whole series of observational parameters with a very high probability of discovering new classes of objects. Our final goal is to acquire and consolidate the skills and competences in view of the development of the entire antenna array of the Ska project, in order to be ready and competitive to lead and participate in the Ska Key Science Projects (Ksp) and to full exploitation of data », Umana concludes.

Featured image: Askap image of the Scorpio field at 912 MHz. The mosaic covers a region of approximately 40 square degrees. The shape of the galactic equator is defined by a series of compact sources and regions of ionized hydrogen (H II regions), associated with star formation sites. Several supernova remnants (Snr) are also visible. Outside the galactic plane some large and bright structures are evident. Among these, the region in the upper center of the field includes the regions H II G345.45 + 1.50 and IC 4628. The white frames are zoom of some representative objects, clockwise, a Snr, a star-forming region with a Massive Young stellar Object, another star-forming region and a pair of Snr. Credits: G. Umana / Inaf

To know more:

  • Read on  Monthly Notices of the Royal Astronomical Society the article ” A first glimpse at the Galactic Plane with the ASKAP: the SCORPIO field “, by G. Umana, C. Trigilio, A. Ingallinera, S. Riggi, F. Cavallaro, J. Marvil, RP Norris, AM Hopkins, CS Buemi, F. Bufano, P. Leto, S. Loru, C. Bordiu, JD Bunton, JD Collier, M. Filipovic, TMO Franzen, MA Thompson, H. Andernach, E . Carretti, S. Dai, A. Kapinska, BS Koribalski, R. Kothes, D. Leahy, D. Mcconnell, N. Tothill and MJ Michałowski
  • Read on  Monthly Notices of the Royal Astronomical Society the article ” Evolutionary map of the Universe (EMU): Compact radio sources in the SCORPIO field towards the galactic plane “, by S Riggi, G Umana, C Trigilio, F Cavallaro, A Ingallinera , P Leto, F Bufano, RP Norris, AM Hopkins, MD Filipović, H Andernach, J Th van Loon, MJ Michałowski, C Bordiu, T An, C Buemi, E Carretti, JD Collier, T Joseph, BS Koribalski, R Kothes , S Loru, D McConnell, M Pommier, E Sciacca, F Schillirò, F Vitello, K Warhurst and M Whiting,
  • Askap is a network of radio telescopes located at the Murchison Radio Astronomy Observatory in the desert region of Western Australia, where the Aboriginal Wajarri Yamatji ethnic group has resided for millennia. Managed and operated by the Australian scientific agency Csiro, Askap has 36 parabolic antennas of 12 meters in diameter with a collection area of ​​4000 square meters: each of the antennas required 13 to 18 hours of assembly. Thirty antennas are arranged in a circle of 2 kilometers in diameter, while the remaining 6 antennas are arranged to form a Reuleaux triangle with a maximum distance from the center of 6 kilometers. Askap is one of the forerunners of the SKA project and has been operational since 2012, but official scientific observations began only from 2020.

Provided by INAF

The Morphology of the X-ray Afterglows and of the Jetted GeV Emission in Long Gamma-ray Bursts (Cosmology)

In a new article published in the Monthly Notices of the Royal Astronomical Society, an ICRA-ICRANet research team (some of them INAF associates) sheds light on the mass and spin of stellar-mass BHs from an extensive analysis of long-duration GRBs

What is the fate of very massive binary stars, which kind of signatures/observables are associated with their stepwise evolution, which kind of new physical laws are revealed, represent the most relevant questions at the heart of relativistic astrophysics. The answer to these questions is intimately related to the explanation of the most powerful transients in the Universe, supernovae (SNe) and gamma-ray bursts (GRBs), and with the formation of neutron star-black hole (NS-BH), of neutron star-neutron star (NS-NS), and possibly BH-BH binaries. A crucial question then arises: how large are the mass and how fast are the rotational spin of those astrophysical BHs and NSs?

A clue to this answer comes out from decades of electromagnetic observations of X-ray binaries in which a BH accretes mass from a stellar companion. From their continuous monitoring, it has turned out that these BH have masses ranging ∼ 5–20 solar masses, where the upper edge is given by the very recently updated mass of the BH harbored by the X-ray binary Cygnus X-1 [1]. While the origin of X-ray binaries is well established, focus is needed to identify the evolutionary channels leading to the onset of GRBs, to their time evolution, as well as to the new physical laws and astrophysical regimes envisaged for their description.

In a new article published in the Monthly Notices of the Royal Astronomical Society [2], an ICRA-ICRANet research team (some of them INAF associates) sheds light on the mass and spin of stellar-mass BHs from an extensive analysis of long-duration GRBs. This has been allowed by fifty years of exponential growth of multiwavelength observations of GRBs and theoretical progress, from which it has been possible to identify the “inner engine” of the GRB, and verify the validity of the BH mass-energy formula established fifty years ago. The subject of study are 380 energetic long GRBs with energy release above 1052 erg in gamma-rays, all with a measured cosmological redshift, and an X-ray afterglow. These systems are accompanied by an SN of type Ic, namely an SN produced by a star which has lost its hydrogen and helium layers. The binary-driven hypernova (BdHN) scenario of long GRBs bridges what we know from binary evolution, with high-energy relativistic astrophysics to explain these extreme systems.

The GRB progenitor system is a binary composed of a carbon-oxygen (CO) star and a companion NS. During their long lifetime, a very massive binary experiences several stages, each one characterized by specific physical phenomena and observables (see left side of Figure 1). The more massive of the two stellar components evolves faster through the nuclear burning phases, leading it to make a first SN explosion, with consequent formation of a NS. Mass-transfer from the ordinary stellar component to the NS leads to an X-ray binary stage. Further binary interactions lead to multiple common envelope phases in which mass loss is enhanced and the ordinary star gets rid of its outer low-density envelope, forming a CO star. The binary orbit shrinks while thermonuclear evolution of the CO star proceeds until its iron core becomes unstable against gravitational collapse, forming a new NS (νNS) at its center, and driving an SN explosion. At this point, a powerful transient starts and its ultimate fate depends crucially on the distance separating the exploding CO star and the NS companion. The SN ejected material triggers a massive accretion process onto the NS companion as well as onto the νNS by matter fallback (see Figure 2).

For compact binaries with orbital periods of the order of 5 minutes (see right side of Figure 1), the companion NS accretes sufficient matter to trigger its gravitational collapse, forming a BH which emanates a distinct, associated emission at high-energies (GeV) characterized by a luminosity as a function of time that follows a power-law. The fallback accretion onto the νNS and its pulsar emission power the GRB X-ray and optical afterglow, characterized by power-law luminosities, different from the one of the GeV emission. BdHNe forming a BH have been called of type I.

From the statistics of the GeV emission, it has been inferred the morphology of the GRBs emission process: it occurs within a conical region of 60◦ measured from the normal to the orbital plane. No GeV radiation is observable outside such a conical region. The X-ray afterglow is instead present in all the BdHN I, independently of the inclination angle of the GRB with respect to the orbital plane. This detailed understanding have allowed the team to infer, from the analysis of the X-ray afterglow, the spin and magnetic field of the νNS. The analysis of the GeV emission have led, for the first time in about fifty years of GRB observations, to directly evaluate the precise mass and spin of the BHs formed in these powerful transients. The specific mass and spin of 11 BHs have been obtained and they range 2.3–8.9 solar masses  and 0.27–0.87 solar masses, respectively.

This treatment of long GRBs, originating from the very massive binary stars, makes ample use of a description based on the four fundamental interactions: relativistic gravity and electrodynamics describe the “inner engine”, weak interactions drive the neutrino emission in the accretion process, and the strong interactions shape the inner structure of the NSs responsible of the X-ray afterglow.

Since the pioneering observations of BATSE instrument on board the Compton satellite [3], we know that GRBs are isotropically distributed when mapped in galactic coordinates. Similarly, following the discovery of their cosmological redshift thanks to BeppoSAX [4], observations of BdHN I have occurred all the way to z = 8.2 (e.g. GRB 090423 [5, 6]). We can safely assert that GRBs, also thanks to their outstanding energetics, have a fundamental role in relativistic astrophysics processes in the 95.5% of our known Universe. Their prolonged emission of polarized synchrotron radiation in the X-rays and in the GeV regime may well have a fundamental role in the life in and of our Universe.

Having said all the above, it comes as a surprise the vision carried forward by the LIGO-Virgo observatories that very massive binary stars should rapidly gravitationally collapse, evolve in into two BHs, crossing the spacetime of our Universe, finally merging into a larger BH. Such a vision avoids the role of any fundamental interactions with the sole exception of gravity, which seems at odds with the field of relativistic astrophysics.

fig. 1: Taken from [7]. Schematic evolutionary path of a massive binary up to the emission of a BdHN. (a) Binary system composed of two main-sequence stars, say 15 and 12 solar masse, respectively. (b) At a given time, the more massive star undergoes the core-collapse SN and forms a NS (which might have a magnetic field B ∼ 10¹³ G). (c) The system enters the X-ray binary phase. (d) The core of the remaining evolved star, rich in carbon and oxygen, for short CO star, is left exposed since the hydrogen and helium envelope have been striped by binary interactions and possibly multiple common-envelope phases (not shown in this diagram). The system is, at this stage, a CO-NS binary, which is taken as the initial configuration of the BdHN model [8]. (e) The CO star explodes as SN when the binary period is of the order of few minutes, the SN ejecta of a few solar masses start to expand and a fast rotating, newborn NS, for short νNS, is left in the center. (f) The SN ejecta accrete onto the NS companion, forming a massive NS (BdHN II) or a BH (BdHN I; this example), depending on the initial NS mass and the binary separation. Conservation of magnetic flux and possibly additional MHD processes amplify the magnetic field from the NS value to B ∼ 10¹⁴ G around the newborn BH. At this stage the system is a νNS-BH binary surrounded by ionized matter of the expanding ejecta. (g) The accretion, the formation and the activities of the BH contribute to the GRB prompt gamma-ray emission and GeV emission. (h) X-ray afterglow powered by the fallback accretion and pulsar-like emission of the νNS. (i) Optical emission of the SN due to nickel decay in the ejecta.
Fig. 2: A SPH simulation from Becerra et al. [8] of the exploding CO-star as the SN in the presence of a companion NS. The CO-star is obtained from the evolution of a 25 solar masses zero-age main-sequence (ZAMS) progenitor which leads to a pre-SN CO-star mass MCO = 6.85 solar masses. The initial mass of the νNS (formed at the center of the SN) is 1.85solar masses and the one of the NS companion is MNS = 2 solar masses. The initial orbital period is 4.8 min. The panels show the mass density on the binary equatorial plane at two selected times from the SN explosion (t = 0 of the simulation), 159 s and 259 s. The reference system is rotated and translated so that the x-axis is along the line that joins the νNS and the NS, and the axis origin (0, 0) is located at the NS position. In this simulation, the NS collapses when it reaches 2.26 solar masses and angular momentum 1.24GM²/c, while the νNS is stable with mass and angular momentum, respectively, 2.04 solar masses and 1.24GM²/c. Up to the final simulation time, the binary system kept bound although the binary orbit widens, reaching an orbital period of 16.5 min and an eccentricity of e = 0.6. The collapse of the NS to the newly-formed BH, characteristic of a BdHN I, occurs at t = 21.6 min.

References: [1] J. C. A. Miller-Jones, A. Bahramian, J. A. Orosz, I. Mandel, L. Gou, T. J. Maccarone, C. J. Neijssel, X. Zhao, J. Zi´o lkowski, M. J. Reid, et al., Science 371, 1046 (2021), 2102.09091. [2] R. Ruffini, R. Moradi, J. A. Rueda, L. Li, N. Sahakyan, Y. C. Chen, Y. Wang, Y. Aimuratov, L. Becerra, C. L. Bianco, et al., MNRAS (2021), 2103.09142. [3] W. S. Paciesas, C. A. Meegan, G. N. Pendleton, M. S. Briggs, C. Kouveliotou, T. M. Koshut, J. P. Lestrade, M. L. McCollough, J. J. Brainerd, J. Hakkila, et al., Astroph. J. Supp. 122, 465 (1999), astro-ph/9903205. [4] M. R. Metzger, S. G. Djorgovski, S. R. Kulkarni, C. C. Steidel, K. L. Adelberger, D. A. Frail, E. Costa, and F. Frontera, Nature (London) 387, 878 (1997). [5] R. Salvaterra, M. Della Valle, S. Campana, G. Chincarini, S. Covino, P. D’Avanzo, A. Fernandez-Soto, C. Guidorzi, F. Mannucci, R. Margutti, et al., Nature (London) 461, 1258 (2009), 0906.1578. [6] R. Ruffini, L. Izzo, M. Muccino, G. B. Pisani, J. A. Rueda, Y. Wang, C. Barbarino, C. L. Bianco, M. Enderli, and M. Kovacevic, Astron. Astroph. 569, A39 (2014), 1404.1840. [7] J. A. Rueda, R. Ruffini, M. Karlica, R. Moradi, and Y. Wang, Astroph. J. 893, 148 (2020), 1905.11339. [8] L. Becerra, C. L. Ellinger, C. L. Fryer, J. A. Rueda, and R. Ruffini, Astroph. J. 871, 14 (2019), 1803.04356.

Provided by ICRA-ICRANet

CDEX Listens to the Sound of Cosmology From a Laboratory Deep Underground (Physics)

Numerous compelling evidences from astroparticle physics and cosmology indicate that the major matter component in the Universe is dark matter, accounting for about 85% with the remaining 15% is the ordinary matter. Nevertheless, people still know little about the dark matter, including its mass and other properties. Many models predict dark matter particles could couple to ordinary particle at weak interaction level, so it is possible to capture the signal of dark matter particle in the direct detection experiment. The scientific goals of the China Dark matter Experiment (CDEX) are on direct detection of light dark matter and neutrino-less double beta decay with p-type point contact germanium (PPCGe) detectors at the China Jinping Underground Laboratory (CJPL). The measurable energy spectra induced by the elastic scattering between dark matter particles and target nucleons in CDEX detector system could give us the information of dark matter mass, spin and other properties.

The analysis of the current dark matter experiments is usually model dependent, and many models beyond the standard model have predicted the existence of dark matter, such as super-symmetry model and extra-dimension model. Due to the variety of physics models, the constraints obtained from same experimental data cannot be applied directly to other models, which brings complications to physical interpretations. Cosmology observations have verified that the major part of dark matter is the non-relativistic cold dark matter, and as a result, the momentum transfer in the scattering process between dark matter particles and nucleons is only about hundreds of MeV, much lower than the electroweak scale (~250 GeV). It is therefore suitable to use effective field theory to analyze the interaction between dark matter and ordinary matter. Two alternative schemes have been proposed in recent years to study different possible interactions, namely non-relativistic effective field theory (NREFT) and chiral effective field theory (ChEFT). An effective theory contains all possible interactions allowed by given symmetric principles, so it can model-independently reduce the complicacy of analysis.

(a) Exclusion limits of different coupling coefficients of NREFT; (b) Exclusion limits of WIMP-pion scattering cross section. ©Science China Press

In the dark matter direct detection experiments, what are mostly focused on are the spin-independent (SI) and spin-dependent (SD) scattering analysis, while EFT can give more momentum-dependent or velocity-dependent interaction which are not taken into consideration usually. Benefiting from the low electrical noise of PCCGe, the analysis threshold of CDEX-1B and CDEX-10 both reach 160 eV, which can largely improve the detection sensitivity for light dark matter.

Based on the data set of CDEX-1B and CDEX-10, CDEX collaboration presents new limits for the couplings of WIMP-nucleon arising from NREFT and ChEFT. In the nonrelativistic effective field theory approach, they improve over the current bounds in the low mχ region. In the chiral effective field theory approach, they for the first time extended the limit on WIMP-pion coupling to the mχ< 6 GeV/c2 region.

Related results have been published online entitled “First experimental constraints on WIMP couplings in the effective field theory framework from CDEX” on Science China-Physics, Mechanics & Astronomy (Sci. China-Phys. Mech. Astron. 64, 281011 (2021))[1]. Prof. Y. F. Zhou from the Institute of Theoretical Physics, Chinese Academy of Sciences wrote a review article for this publication[2].

The operation and analysis of CDEX-1B and CDEX-10 are coming to the end, and the next generation of experiments CDEX-100/CDEX-1T are under preparation now. The lower background level and improvement of PPCGe performance can raise the sensitivity of direct detection experiment. While the next generation experiment of CDEX can discover dark matter remains unknown, but the mystery of dark matter will encourage more and more researchers to pursue its studies until the day when this profound mystery of the Universe will be solved.

Featured image: The schematic setup of the next generation CDEX experiment in CJPL-II © Science China Press

See the articles:

[1] Y. Wang et al., (CDEX Collaboration), First experimental constraints on WIMP couplings in the effective field theory framework from CDEX, Sci. China-Phys. Mech. Astron. 64, 281011 (2021), https://doi.org/10.1007/s11433-020-1666-8 [2] Y.-F. Zhou, Improved constraints on dark matter effective interactions from CDEX, Sci. China-Phys. Mech. Astron. 64, 2841031 (2021), https://doi.org/10.1007/s11433-021-1679-4

Provided by Science China Press

Why We Prefer Concave Shape Inflation Potential Rather Than Convex? (Cosmology / Quantum / Maths)


⦿ There are two types of inflation potential we generally prefer: Concave and convex. The Planck data on cosmic microwave background indicates that the Starobinsky-type model with concave inflation potential is favored over the convex-type chaotic inflation. But why? This reason is still unclear.

⦿ Now, Chen and Yeom investigated Euclidean wormholes in the context of the inflationary scenario in order to answer the question on the preference of a specific shape of the inflaton potential.

⦿ They argued that if our universe began with a Euclidean wormhole, then the Starobinsky-type inflation is probabilistically favored.

⦿ They showed that only one end of the wormhole can be classicalized for a convex potential, while both ends can be classicalized for a concave potential. The latter is therefore more probable.

⦿ Their study point towards the fact that its not the universe but the wormhole which is expanding

How did the universe begin? This has long been one of the most fundamental questions in physics. The Big Bang scenario, when tracing back to the Planck time, indicates that the universe should start from a regime of quantum gravity that is describable by a wave function of the universe governed by the Wheeler-DeWitt (WDW) equation. The WDW equation is a partial differential equation and hence it requires a boundary condition. This boundary condition allows one to assign the probability of the initial condition of our universe. As is well known, to overcome some drawbacks of the Big Bang scenario, an era of inflation has been introduced. Presumably, the boundary condition of the WDW equation would dictate the nature of the inflation.

The Planck data on cosmic microwave background (CMB) indicates that certain inflation models are more favored than some others. In particular, the Starobinsky-type model with concave inflation potential (V” < 0 when the inflation is dominant.) appears to be favored over the convex-type (V” > 0) chaotic inflation. Is there any reason for this? Now, Chen and Yeom argued that if our universe began with a Euclidean wormhole, then the Starobinsky-type inflation is probabilistically favored.

One reasonable assumption for the boundary condition of the WDW equation was suggested by Hartle and Hawking, where the ground state of the universe is represented by the Euclidean path integral between two hypersurfaces. The Euclidean propagator can be described as follows:

where gµν is the metric, φ is an inflaton field, SE is the Euclidean action, and h (Sys. (a,b), stat. (µν)) and χ^a,b are the boundary values of gµν and φ on the initial (say, a) and the final (say, b) hypersurfaces, respectively. Using the steepestdescent approximation, this path integral can be well approximated by a sum of instantons, where the probability of each instanton becomes P ∝ e^−S_E. This approach has been applied to different issues with success: (1) It is consistent with the WKB approximation, (2) It has good correspondences with perturbative quantum field theory in curved space, (3) It renders correct thermodynamic relations of black hole physics and cosmology. These provide Chen and Yeom the confidence that the eventual quantum theory of gravity should retain this notion as an effective description.

FIG. 1: Complex time contour and numerical solution of ar, ai, φr, and φi for Vch. The upper figure is a physical interpretation about the wormhole, where Part A (red) and C (green) are Lorentzian and Part B (blue) is Euclidean. © Chen and Yeom

In their original proposal, Hartle and Hawking considered only compact instantons. In that case it is proper to assign the condition for only one boundary; this is the so-called no-boundary proposal. In general, however, the path integral should have two boundaries. If the arrow of time is symmetric between positive and negative time for classical histories, then one may interpret this situation as having two universes created from nothing, where the probability is determined by the instanton that connects the two classical universes. Such a process can be well described by the Euclidean wormholes.

Now, Chen and Yeom investigated Euclidean wormholes in the context of the inflationary scenario in order to answer the question on the preference of a specific shape of the inflaton potential. They showed that only one end of the wormhole can be classicalized for a convex potential, while both ends can be classicalized for a concave potential. The latter is therefore more probable.

“We investigated Euclidean wormholes with a non-trivial inflaton potential. We showed that in terms of probability, the Euclidean path-integral is dominated by Euclidean wormholes, and only the concave potential explains the classicality of Euclidean wormholes. This helps to explain, in our view, why our universe prefers the Starobinsky-like model rather than the convex-type chaotic inflation model.

— told Chen, first author of the study

It should be mentioned that there exist other attempts to explain the origin of the concave inflation potential. For example, it was reported by Hertog in his paper that, the Starobinsky-like concave potential is preferred if a volume-weighted term is added to the measure. Chen and Yeom note that the same principle can be applied not only to compact instantons but also to Euclidean wormholes; hence, this proposal may support their result as well. They must caution, however, that the justification of such a volume-weighted term is theoretically subtle.

This is of course not the end of the story. One needs to further investigate whether this Euclidean wormhole methodology is compatible with other aspects of inflation. It will also be interesting to explore the relation between the probability distribution of wormholes and the detailed shapes of various inflaton potentials. Furthermore, if this Euclidean wormhole creates any bias from the Bunch-Davies state, then it may in principle be confirmed or falsified by future observations. They left these topics for future investigations.

Featured image: Comparing the inflation models with the observational constraints. © Ke Wang

Reference: Chen, P., Yeom, Dh. Why concave rather than convex inflaton potential?. Eur. Phys. J. C 78, 863 (2018). https://doi.org/10.1140/epjc/s10052-018-6357-0 https://link.springer.com/article/10.1140/epjc/s10052-018-6357-0

Copyright of this article totally belongs to our author S. Aman. One is allowed to reuse it only by giving proper credit either to him or to us

The Very First Structures in the Universe (Cosmology / Astronomy)

Astrophysicists at the Universities of Göttingen and Auckland simulate microscopic clusters from the Big Bang

The very first moments of the Universe can be reconstructed mathematically even though they cannot be observed directly. Physicists from the Universities of Göttingen and Auckland (New Zealand) have greatly improved the ability of complex computer simulations to describe this early epoch. They discovered that a complex network of structures can form in the first trillionth of a second after the Big Bang. The behaviour of these objects mimics the distribution of galaxies in today’s Universe. In contrast to today, however, these primordial structures are microscopically small. Typical clumps have masses of only a few grams and fit into volumes much smaller than present-day elementary particles. The results of the study have been published in the journal Physical Review D.

The researchers were able to observe the development of regions of higher density that are held together by their own gravity. “The physical space represented by our simulation would fit into a single proton a million times over,” says Professor Jens Niemeyer, head of the Astrophysical Cosmology Group at the University of Göttingen. “It is probably the largest simulation of the smallest area of the Universe that has been carried out so far.” These simulations make it possible to calculate more precise predictions for the properties of these vestiges from the very beginnings of the Universe.

Professor Jens Niemeyer. Photo: University of Göttingen

Although the computer-simulated structures would be very short-lived and eventually “vaporise” into standard elementary particles, traces of this extreme early phase may be detectable in future experiments. “The formation of such structures, as well as their movements and interactions, must have generated a background noise of gravitational waves,” says Benedikt Eggemeier, a PhD student in Niemeyer’s group and first author of the study. “With the help of our simulations, we can calculate the strength of this gravitational wave signal, which might be measurable in the future.”

It is also conceivable that tiny black holes could form if these structures undergo runaway collapse. If this happens they could have observable consequences today, or form part of the mysterious dark matter in the Universe. “On the other hand,” says Professor Easther, “If the simulations predict black holes form, and we don’t see them, then we will have found a new way to test models of the infant Universe.”

Featured image: The results of the simulation show the growth of tiny, extremely dense structures very soon after the inflation phase of the very early universe. Between the initial and final states in the simulation (top left and right respectively), the area shown has expanded to ten million times its initial volume, but is still many times smaller than the interior of a proton. The enlarged clump at the bottom left would have a mass of about 20kg. Photo: Jens Niemeyer, University of Göttingen

Original publication:Eggemeier B et al, Formation of inflation halos after inflation. Physical Review D (2021). DoI: 10.1103/PhysRevD.103.063525

Provided by University of Gottingen

Large Proto-Cluster Of Galaxies Discovered In The Midst Of Clearing The Cosmic Fog (Astronomy)

When the universe was about 350 million years old it was dark: there were no stars or galaxies, only neutral gas—mainly hydrogen—the residue of the Big Bang. That foggy period began to clear as atoms clumped together to form the first stars and the first quasars, causing the gas to ionize and high-energy photons to travel freely through space.

This epoch, called the “reionization” epoch, lasted about 370 million years and the first large structures in the universe appear as groups or clusters of galaxies.

An international team of astronomers grouped in the LAGER consortium (Lyman Alpha Galaxies in the Epoch of Reionization), integrated by Leopoldo Infante, Director of Carnegie’s Las Campanas Observatory, and postdoctoral researcher Jorge González-López, discovered the most-distant high-density cluster of galaxies, or protocluster, ever observed. This study, published in Nature, opens new avenues for understanding the evolution of high-density regions in the universe and the galaxies of which they are composed.

“We have found a protocluster observed when the universe was less than 6 percent of its present age, near the end of the reionization period. It is the most distant protocluster so far confirmed with spectroscopy. An estimate of the mass involved suggests that for the present epoch, it would be a massive cluster of galaxies similar to the famous Coma cluster in the nearby universe,” Infante explained.

The Dark Energy Camera (DECam), mounted on the 4-meter Victor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory (CTIO), which is a program of NSF’s NOIRLab and is located in Chile, was used to carry out the research.

Confirmation of the candidate galaxies was then achieved with spectra obtained with the 6.5-meter Magellan telescopes at Las Campanas Observatory, along with careful data reduction and analysis.

The sky conditions at Las Campanas Observatory, Infante emphasized, allow for deep, high-resolution observations of very faint objects.

“The Magellan telescopes, with their active optics and extremely sensitive spectrographs, allow us to observe galaxies whose light was emitted as early as 750 million years after the Big Bang,” Infante stressed.

The LAGER survey seeks to understand physics at the time of reionization, but in the context of galaxy formation and evolution.

“This research is important because it establishes the conditions of matter in the universe at the time of reionization, when galaxies formed. The discovery of the protocluster makes it possible not only to study individual galaxies, but also to understand how clusters and structures form in the universe. At the same time, it reveals the initial conditions for the formation of structures,” Infante added.

So far, the LAGER study has discovered dozens of galaxies whose light was emitted when the universe was about 750 million years old. To understand the physical conditions of matter in the universe at those ages, researchers need to multiply the number of galaxies observed by a factor of at least 10.

“We will continue to examine more of these galaxies using the Blanco 4-meter and Magellan 6.5-meter telescopes until we reach the necessary statistical precision. We are confident that in the process we will find many other interesting objects like the protocluster discovered in this work,” concluded the LCO director.

Featured image: 3D spatial distribution of 16 Spectroscopically confirmed proto-clusters © Carnegie Science

Reference: Hu, W., Wang, J., Infante, L. et al. A Lyman-α protocluster at redshift 6.9. Nat Astron (2021). https://www.nature.com/articles/s41550-020-01291-y https://doi.org/10.1038/s41550-020-01291-y

Provided by Carnegie Institution for Science

Universe is e-dimensional, Not 3-Dimensional (Cosmology / Astronomy)

Subhash Kak and colleagues in their very recent paper proposed that space is not three-dimensional instead it is e-dimensional. Where, ‘e’ here is Euler’s number and its value runs 2.71828.

Efficiency of dimensions for d = 2, e, 3, and 4.© S. Kak et al.

Guys, we all know that universe is expanding and how do we measure this expansion? Yes right, with the help of Hubble constant. We can measure it either from ‘early’ universe to ‘present’ or from ‘late’ to ” present. There are two conflicting values of the Hubble constant, based on whether one analyzes the cosmic microwave background (CMB) (the “early” universe estimate) or observes motions of stars and galaxies (the “late” universe estimate). According to the Planck Collaboration exploring the early universe, H0 is about 67 km s−¹ Mpc−¹, whereas a late universe estimate, Supernova H0 for the Equation of State (SHoES), has it at about 74 km s−¹ Mpc−¹. Yeah right, we didn’t got same value of rate of expansion from our past observations.

This creates the discrepancy between the early and late universe estimates of the Hubble constant has not yielded to any analysis and it has been termed a crisis in physics.

Now, Subhash Kak and colleagues presented a resolution to the problem of the diverging estimates of the Hubble constant H0, based on early- or late- universe models. They proposed e-dimensional hypothesis and applied this idea to the question of expansion rate of the universe. Specifically, they considered the problem of the discrepancy of the Hubble constant based on early and late universe models and showed that it solves the problem of the discrepancy.

According to this hypothesis, “the early universe model provides the true e-dimensional estimate whereas the late universe model imposes a 3-dimensional gloss on our measurements, which arises from the mathematical models that are fitted into the actual measurements.”

Dr. Kak in his paper states that, ‘our cognitions are based on counting, we associate the nearest integer space of 3 dimensions to space which leads to our normal sense about its nature.’

According to Dr. Kak, the dimensionality of such a universe shall be ‘quite close to the fractal dimension associated with the Menger sponge’ which he points out ‘has a dimension of roughly 2.727.’ Where, a fractal is a shape that is self-replicating and scale-invariant and the Menger sponge is a deterministic fractal with dimension close to ‘e’.

References: Kak, S et al. Information theory and dimensionality of space. Sci Rep 10, 20733 (2020). https://www.nature.com/articles/s41598-020-77855-9 https://doi.org/10.1038/s41598-020-77855-9

This article is originally written by S. Aman. One is allowed to reuse this article only by giving proper credit either to author S. Aman or to us.

Connecting Emptiness (Cosmology / Astronomy)

In searching for a robust statistical method to map large regions of almost empty space formed alongside the large-scale structure of our Universe, researchers of the Cosmostatistics Initiative (COIN) found inspiration in the field of criminology.

From left to right, data projection form the 2df Galaxy survey, visualization of the Illustris simulation, spatial distribution of registered crimes in the city of Chicago. © Cosmostatistics Initiative

At very large scales, the matter in our Universe is distributed in filamentary structures, regions of high density holding galaxy clusters and superclusters. These structures live side by side with large regions of almost complete emptiness known as ‘voids’. This lattice structure can be easily recognized in both spectroscopic surveys and results from N-body simulations. Although detecting and studying high-density regions in these maps may be more appealing to the researcher from an astrophysical point of view, voids can also be quite informative from a cosmological perspective.

Identified ridges found by the CRP #6 team (purple) overlaid with those identified by an alternative method. © Cosmostatistics Initiative

Given that the same underlying cosmology which generated large-scale structure is responsible for the formation of regions of low matter density, understanding the statistics and dynamics of cosmic voids can impose constraints on the parameters of cosmological models. This provides information on the evolution of matter arrangement throughout the history of the Universe as well as on the nature of dark energy, while avoiding the need to model complex gravitational effects. Even so, the detection of these voids, or their 2D projections called ‘troughs’, is a challenging task. The main difficulty being that there is no clear definition of what constitutes a void, or how much emptiness is enough to characterize them.

This data-intensive challenge is not very different from the one encountered in criminology when trying to map regions exhibiting high crime incidence. By analyzing the data of reported crimes, experts aim at identifying dangerous zones to optimize patrol routes along incident-dense regions (see Figure 1). During a few hot summer days in the shadow of Mont Blanc, the team of the 6th COIN Residency Program judged that there were enough similarities between these scenarios, as well as previous related work, to encourage an investigation of the method’s performance when faced with current astronomical data.

The publicly available implementation of an algorithm, called subspace-constrained mean shift, was adapted and optimized to estimate regions of high density in mass maps from the first-year data release of the Dark Energy Survey.

Moreover, the CRP6 research team performed tests of statistical consistency using simulations with different levels of noise, and compared results to previous research and alternative techniques, providing a clear picture of the method’s applicability. Results from both simulated and real data showed that the upgraded algorithm can consistently find connected filamentary structures that follow the same patterns of mass distribution (see Figure 2). The latter, in turn, enable the identification of troughs as underdense regions delimited by ridges. A desirable consequence of this approach is that the emerging troughs can have any shape, as opposed to methods that limit their morphology.

With the proposed tool, the team produced a catalog of troughs, which will be made publicly available for other groups to impose additional constraints on cosmological models.

Reference: Ben Moews, Morgan A Schmitz, Andrew J Lawler, Joe Zuntz, Alex I Malz, Rafael S de Souza, Ricardo Vilalta, Alberto Krone-Martins, Emille E O Ishida, for the COIN Collaboration, Ridges in the Dark Energy Survey for cosmic trough identification, Monthly Notices of the Royal Astronomical Society, Volume 500, Issue 1, January 2021, Pages 859–870, https://doi.org/10.1093/mnras/staa3204&nbsp;Moews et al., 2020 – MNRAS

Provided by COIN