Tag Archives: #expansion

Accelerated Expansion of the Universe Is Due To Spacetime Vorticity (Cosmology /Astronomy)

Babur Mirza and colleagues presented a general relativistic mechanism for accelerated cosmic expansion and the Hubble’s constant. They showed that spacetime vorticity coupled to the magnetic field density in galaxies causes the galaxies to recede from one another at a rate equal to the Hubble’s constant.

Accelerated expansion of the universe, as observed, for example, in the cosmological redshift measurements using type-Ia supernovae (SNe Ia) as standard candles, implies the need for an expansion energy effective at least up to the Mpc scale. A number of independent observations (including the SNe Ia redshift, the Hubble’s constant measurements, the cosmic microwave background (CMB), baryon acoustic oscillations, and various cosmological probes), have measured the contributions of matter and the cosmological constant to the energy density of the universe, providing an accurate measurement of the cosmic acceleration. However the amount of energy for this acceleration implies a hidden or dark form of energy which is approximately three times of the observed gravitational mass-energy density in the universe.

Within Einstein’s general theory of relativity, the observed expansion rate can be accounted for by including a cosmological constant, whose origin remains somewhat mysterious. In this context various mechanisms have been postulated, including new forms of hypothetical particles, or modifications of the Newtonian-Einsteinian law of gravitation at large distances, among others. However these theories are specialized in the sense that they fail to account for other observed features of the universe, such as the high degree of isotropy in CMB, or even some feature of the expansion, such as the correct value of the Hubble’s constant.

Now, Babur and colleagues in their work showed that the specific form of the cosmological constant, hence cosmic acceleration, which can be described by spacetime vorticity, is generated by galactic rotations. They showed that this vorticity coupled to the local (galactic) magnetic field provides the requisite push (repulsive energy) causing the individual galaxies to recede at an accelerated rate. They are therefore led to an oscillatory universe, where expansion and conversely contraction rate is determined by local spacetime vorticity, rather than global geometry (curvature) of the spacetime.

“To recapitulate we remark that, within the above model of the accelerated expansion of the universe, local spacetime vorticity and magnetic field energy generation within galaxies and galactic clusters act as the feedback mechanism for expansion. Thus, contrary to some recent suggestions that accelerated expansion must imply a violation of the law of conservation of energy, we see that energy conservation remains strictly valid not only locally but globally as well. The continued universal acceleration depends on the energy generation within galaxies, which in turn is determined by accretion rate in galactic nuclei.”, said Babur.

Friends, conversion of matter-energy density into the magnetic field energy under such conditions can only take a finite amount of time, hence the magnetic field driven acceleration cannot continue indefinitely for a finite total mass. Since the acceleration aB ∼ B’², where B’² is the magnetic energy density per unit volume, they saw that with the decreasing feedback magnetic field, universal acceleration after reaching an maximum will gradually decrease. With the decrease of Magnetic energy generation via accretion, a gradual deacceleration under gravitational attraction is likely to cause cosmic contraction. They therefore proposed that they have an oscillatory universe, where magneto-vorticity coupling rather than global spacetime curvature causes the expansion and contraction phases.

According to Babur, “A very high degree of entropy must have existed at the early stage of the universe, as inferred from the Planckian shape of the CMB radiation. This raises the paradox for other cosmological models, since entropy should decrease closer to the initial singularity (big bang). Our model implies that this must be so due to the expansion started before the cross-over R = Rs, where Rs is Schwarzschild singularity. Subsequently, as this expansion (inflation) stops, and matter formation starts, expansion under spacetime vorticity must now cause matter entropy to gradually increase with time. As deduced in our paper, this explains the high isotropy and the Planckian profile of the CMB spectrum, carrying the imprint of this initial inflation over R > Rs.”

Reference: Babur M. Mirza, “Can accelerated expansion of the universe be due to spacetime vorticity?”, Modern Physics Letters A, Vol. 33, No. 40, 1850240 (2018). https://www.worldscientific.com/doi/abs/10.1142/S0217732318502401 https://doi.org/10.1142/S0217732318502401

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.

No Dark Energy, Dark Matter Pressure Itself Helps In Expansion Of Universe (Cosmology / Astronomy)

Zenaib Rezaei and colleagues explored the expansion dynamics of the Universe in the presence of dark matter pressure. Their results verify that the dark matter pressure leads to the higher values of the Hubble parameter at each redshift and the expansion of the Universe grows due to the DM pressure.

Expansion dynamics of the Universe is one of the important subjects in Modern cosmology. The dark energy equation of state determines this dynamics so that the Universe is in an accelerating phase. However, the dark matter can also affect the accelerated expansion of the Universe through its equation of state. So, Zenaib Rezaei and colleagues, in their present work, explored the expansion dynamics of the Universe in the presence of dark matter pressure.

“Our results verify that the dark matter pressure leads to the higher values of the Hubble parameter at each redshift and the expansion of the Universe grows due to the DM pressure.”, said Zenaib.

FIG. 2: Evolution of DM density, ρDM(a), versus the cosmological scale factor, a, for two cases of zero pressure DM (ZPDM) and non zero pressure DM (NZPDM).

They employed dark matter (DM) equation of state from the observational data of the rotational curves of galaxies to investigate the accelerated expansion of the Universe in the presence of the DM pressure. Their results verify that at lower values of the scale factor, the existence of the DM pressure leads to the larger DM density. They also found that the Hubble parameter also has higher values when the DM pressure is considered.

Their calculations confirmed that the DM pressure results in the increase of the growth rate of the Hubble parameter with the redshift. Moreover, the growth of the scale factor versus the cosmic time is more significant when the DM pressure is present. In addition, they have shown that the luminosity distance and distance moduli are not considerably influenced by the DM pressure. Besides, their results indicated that the DM pressure affects the deceleration parameter.

Reference: Zeinab Rezaei, “Accelerated expansion of the Universe in the Presence of Dark Matter Pressure”, Canadian Journal of Physics, pp. 1-17, 19 June 2019, https://cdnsciencepub.com/doi/10.1139/cjp-2019-0135 https://doi.org/10.1139/cjp-2019-0135

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.

Astronomers Get Their Wish, and a Cosmic Crisis Gets Worse (Cosmology / Astronomy)

We don’t know why the universe appears to be expanding faster than it should. New ultra-precise distance measurements have only intensified the problem.

On December 3, humanity suddenly had information at its fingertips that people have wanted for, well, forever: the precise distances to the stars.

The Gaia telescope gauges the distances to stars by measuring their parallax, or apparent shift over the course of a year. Closer stars have a larger parallax. © Samuel Velasco/Quanta Magazine

“You type in the name of a star or its position, and in less than a second you will have the answer,” Barry Madore, a cosmologist at the University of Chicago and Carnegie Observatories, said on a Zoom call last week. “I mean …” He trailed off.

“We’re drinking from a firehose right now,” said Wendy Freedman, also a cosmologist at Chicago and Carnegie and Madore’s wife and collaborator.

“I can’t overstate how excited I am,” Adam Riess of Johns Hopkins University, who won the 2011 Nobel Prize in Physics for co-discovering dark energy, said in a phone call. “Can I show you visually what I’m so excited about?” We switched to Zoom so he could screen-share pretty plots of the new star data.

The data comes from the European Space Agency’s Gaia spacecraft, which has spent the past six years stargazing from a perch 1 million miles high. The telescope has measured the “parallaxes” of 1.3 billion stars — tiny shifts in the stars’ apparent positions in the sky that reveal their distances. “The Gaia parallaxes are by far the most accurate and precise distance determinations ever,” said Jo Bovy, an astrophysicist at the University of Toronto.

Best of all for cosmologists, Gaia’s new catalogue includes the special stars whose distances serve as yardsticks for measuring all farther cosmological distances. Because of this, the new data has swiftly sharpened the biggest conundrum in modern cosmology: the unexpectedly fast expansion of the universe, known as the Hubble tension.

The tension is this: The cosmos’s known ingredients and governing equations predict that it should currently be expanding at a rate of 67 kilometers per second per megaparsec — meaning we should see galaxies flying away from us 67 kilometers per second faster for each additional megaparsec of distance. Yet actual measurements consistently overshoot the mark. Galaxies are receding too quickly. The discrepancy thrillingly suggests that some unknown quickening agent may be afoot in the cosmos.

“It would be incredibly exciting if there was new physics,” Freedman said. “I have a secret in my heart that I hope there is, that there’s a discovery to be made there. But we want to make sure we’re right. There’s work to do before we can say so unequivocally.”

That work involves reducing possible sources of error in measurements of the cosmic expansion rate. One of the biggest sources of that uncertainty has been the distances to nearby stars — distances that the new parallax data appears to all but nail down.

In a paper posted online last night and submitted to The Astrophysical Journal, Riess’s team has used the new data to peg the expansion rate at 73.2 kilometers per second per megaparsec, in line with their previous value, but now with a margin of error of just 1.8%. That seemingly cements the discrepancy with the far lower predicted rate of 67.

Freedman and Madore expect to publish their group’s new and improved measurement of the cosmic expansion rate in January. They too expect the new data to firm up, rather than shift, their measurement, which has tended to land lower than Riess’s and those of other groups but still higher than the prediction.

Since Gaia launched in December 2013, it has released two other massive data sets that have revolutionized our understanding of our cosmic neighborhood. Yet Gaia’s earlier parallax measurements were a disappointment. “When we looked at the first data release” in 2016, Freedman said, “we wanted to cry.”

An Unforeseen Problem

If parallaxes were easier to measure, the Copernican revolution might have happened sooner.

Copernicus proposed in the 16th century that the Earth revolves around the sun. But even at the time, astronomers knew about parallax. If Earth moved, as Copernicus held, then they expected to see nearby stars shifting in the sky as it did so, just as a lamppost appears to shift relative to the background hills as you cross the street. The astronomer Tycho Brahe didn’t detect any such stellar parallax and thereby concluded that Earth does not move.

And yet it does, and the stars do shift — albeit barely, because they’re so far away.

It took until 1838 for a German astronomer named Friedrich Bessel to detect stellar parallax. By measuring the angular shift of the star system 61 Cygni relative to the surrounding stars, Bessel concluded that it was 10.3 light-years away. His measurement differed from the true value by only 10% — Gaia’s new measurements place the two stars in the system at 11.4030 and 11.4026 light-years away, give or take one or two thousandths of a light-year.

The 61 Cygni system is exceptionally close. More typical Milky Way stars shift by mere ten-thousandths of an arcsecond — just hundredths of a pixel in a modern telescope camera. Detecting the motion requires specialized, ultra-stable instruments. Gaia was designed for the purpose, but when it switched on, the telescope had an unforeseen problem.

The telescope works by looking in two directions at once and tracking the angular differences between stars in its two fields of view, explained Lennart Lindegren, who co-proposed the Gaia mission in 1993 and led the analysis of its new parallax data. Accurate parallax estimates require the angle between the two fields of view to stay fixed. But early in the Gaia mission, scientists discovered that it does not. The telescope flexes slightly as it rotates with respect to the sun, introducing a wobble into its measurements that mimics parallax. Worse, this parallax “offset” depends in complicated ways on objects’ positions, colors and brightness.

However, as data has accrued, the Gaia scientists have found it easier to separate the fake parallax from the real. Lindegren and colleagues managed to remove much of the telescope’s wobble from the newly released parallax data, while also devising a formula that researchers can use to correct the final parallax measurements depending on a star’s position, color and brightness.

Climbing the Ladder

With the new data in hand, Riess, Freedman and Madore and their teams have been able to recalculate the universe’s expansion rate. In broad strokes, the way to gauge cosmic expansion is to figure out how far away distant galaxies are and how fast they’re receding from us. The speed measurements are straightforward; distances are hard.

The most precise measurements rely on intricate “cosmic distance ladders.” The first rung consists of “standard candle” stars in and around our own galaxy that have well-defined luminosities, and which are close enough to exhibit parallax — the only sure way to tell how far away things are without traveling there. Astronomers then compare the brightness of these standard candles with that of fainter ones in nearby galaxies to deduce their distances. That’s the second rung of the ladder. Knowing the distances of these galaxies, which are chosen because they contain rare, bright stellar explosions called Type 1a supernovas, allows cosmologists to gauge the relative distances of farther-away galaxies that contain fainter Type 1a supernovas. The ratio of these faraway galaxies’ speeds to their distances gives the cosmic expansion rate.

Parallaxes are thus crucial to the whole construction. “You change the first step — the parallaxes — then everything that follows changes as well,” said Riess, who is one of the leaders of the distance ladder approach. “If you change the precision of the first step, then the precision of everything else changes.”

Riess’s team has used Gaia’s new parallaxes of 75 Cepheids — pulsating stars that are their preferred standard candles — to recalibrate their measurement of the cosmic expansion rate.

Freedman and Madore, Riess’s chief rivals at the top of the distance ladder game, have argued in recent years that Cepheids foster possible missteps on higher rungs of the ladder. So rather than lean too heavily on them, their team is combining measurements based on multiple kinds of standard-candle stars from the Gaia data set, including Cepheids, RR Lyrae stars, tip-of-the-red-giant-branch stars and so-called carbon stars.

“Gaia’s [new data release] is providing us with a secure foundation,” said Madore. Although a series of papers by Madore and Freedman’s team aren’t expected for a few weeks, they noted that the new parallax data and correction formula appear to work well. When used with various methods of plotting and dissecting the measurements, data points representing Cepheids and other special stars fall neatly along straight lines, with very little of the “scatter” that would indicate random error.

“It’s telling us we’re really looking at the real stuff,” Madore said.

This article is originally written by Natalie Wolchover and is republished here from quanta magazine under common creative licenses.

Multi-messenger Astronomy Offers New Estimates of Neutron Star Size And Universe Expansion (Planetary Science)

Study finds neutron stars are typically about 11.75 kilometers in radius, and provides a novel calculation of the Hubble constant.

A combination of astrophysical measurements has allowed researchers to put new constraints on the radius of a typical neutron star and provide a novel calculation of the Hubble constant that indicates the rate at which the universe is expanding.

Collision of two neutron stars showing the electromagnetic and gravitational-wave emissions during the merger process. The combined interpretation of multiple messengers allows astrophysicists to understand the internal composition of neutron stars and to reveal the properties of matter under the most extreme conditions in the universe. © Tim Dietrich

“We studied signals that came from various sources, for example recently observed mergers of neutron stars,” said Ingo Tews, a theorist in Nuclear and Particle Physics, Astrophysics and Cosmology group at Los Alamos National Laboratory, who worked with an international collaboration of researchers on the analysis to appear in the journal Science on December 18. “We jointly analyzed gravitational-wave signals and electromagnetic emissions from the mergers, and combined them with previous mass measurements of pulsars or recent results from NASA’s Neutron Star Interior Composition Explorer. We find that the radius of a typical neutron star is about 11.75 kilometers and the Hubble constant is approximately 66.2 kilometers per second per megaparsec.”

Combining signals to gain insight into distant astrophysical phenomena is known in the field as multi-messenger astronomy. In this case, the researchers’ multi-messenger analysis allowed them to restrict the uncertainty of their estimate of neutron star radii to within 800 meters.

Their novel approach to measuring the Hubble constant contributes to a debate that has arisen from other, competing determinations of the universe’s expansion. Measurements based on observations of exploding stars known as supernovae are currently at odds with those that come from looking at the Cosmic Microwave Background (CMB), which is essentially the left over energy from the Big Bang. The uncertainties in the new multimessenger Hubble calculation are too large to definitively resolve the disagreement, but the measurement is slightly more supportive of the CMB approach.

Tews’ primary scientific role in the study was to provide the input from nuclear theory calculations that are the starting point of the analysis. His seven collaborators on the paper comprise an international team of scientists from Germany, the Netherlands, Sweden, France, and the United States.

A combination of astrophysical measurements has allowed researchers to put novel constraints on the radius of a typical neutron star and provide a new calculation of the Hubble constant that indicates the rate at which the universe is expanding.

“We studied signals that came from various sources, for example recently observed mergers of neutron stars,” said Ingo Tews, a theorist in Nuclear and Particle Physics, Astrophysics and Cosmology group at Los Alamos National Laboratory, who worked with an international collaboration of researchers on the analysis to appear in the journal Science on December 18. “We jointly analyzed gravitational-wave signals and electromagnetic emissions from the mergers, and combined them with previous mass measurements of pulsars or recent results from NASA’s Neutron Star Interior Composition Explorer. We find that the radius of a typical neutron star is about 11.75 kilometers and the Hubble constant is approximately 66.2 kilometers per second per megaparsec.”

Combining signals to gain insight into distant astrophysical phenomena is known in the field as multi-messenger astronomy. In this case, the researchers’ multi-messenger analysis allowed them to restrict the uncertainty of their estimate of neutron star radii to within 800 meters.

Their novel approach to measuring the Hubble constant contributes to a debate that has arisen from other, competing determinations of the universe’s expansion. Measurements based on observations of exploding stars known as supernovae are currently at odds with those that come from looking at the Cosmic Microwave Background (CMB), which is essentially the left over energy from the Big Bang. The uncertainties in the new multimessenger Hubble calculation are too large to definitively resolve the disagreement, but the measurement is slightly more supportive of the CMB approach.

Tews’ primary scientific role in the study was to provide the input from nuclear theory calculations that are the starting point of the analysis. His seven collaborators on the paper comprise an international team of scientists from Germany, the Netherlands, Sweden, France, and the United States.

References: Tim Dietrich, Michael W. Coughlin, Peter T. H. Pang, Mattia Bulla, Jack Heinzel, Lina Issa, Ingo Tews, Sarah Antier, “Multimessenger constraints on the neutron-star equation of state and the Hubble constant”, Science 18 Dec 2020: Vol. 370, Issue 6523, pp. 1450-1453 DOI: 10.1126/science.abb4317 https://science.sciencemag.org/content/370/6523/1450

Provided by Los Alamos National Laboratory

About Los Alamos National Laboratory

Los Alamos National Laboratory, a multidisciplinary research institution engaged in strategic science on behalf of national security, is managed by Triad, a public service oriented, national security science organization equally owned by its three founding members: Battelle Memorial Institute (Battelle), the Texas A&M University System (TAMUS), and the Regents of the University of California (UC) for the Department of Energy’s National Nuclear Security Administration.

Los Alamos enhances national security by ensuring the safety and reliability of the U.S. nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction, and solving problems related to energy, environment, infrastructure, health, and global security concerns.

Hubble Observes Spectacular Supernova Time-Lapse (Astronomy)

The NASA/ESA’s Hubble Space Telescope has tracked the fading light of a supernova in the spiral galaxy NGC 2525, located 70 million light years away. Supernovae like this one can be used as cosmic tape measures, allowing astronomers to calculate the distance to their galaxies. Hubble captured these images as part of one of its major investigations, measuring the expansion rate of the Universe, which can help answer fundamental questions about our Universe’s very nature.

Pictured here is part of the captivating galaxy NGC 2525. Located nearly 70 million light-years from Earth, this galaxy is part of the constellation of Puppis in the southern hemisphere. Together with the Carina and the Vela constellations, it makes up an image of the Argo from ancient greek mythology. On the left, a brilliant supernova is clearly visible in the image. The supernova is formally known as SN2018gv and was first spotted in mid-January 2018. The NASA/ESA Hubble Space Telescope captured the supernova in NGC 2525 as part of one of its major investigations; measuring the expansion rate of the Universe, which can help answer fundamental questions about our Universe’s very nature. Supernovae like this one can be used as cosmic tape measures, allowing astronomers to calculate the distance to their galaxies. ESA/Hubble has now published a unique time-lapse of this galaxy and it’s fading supernova.
CREDIT: ESA/Hubble & NASA, A. Riess and the SH0ES team Acknowledgment: Mahdi Zamani

The supernova, formally known as SN2018gv, was first spotted in mid-January 2018. The NASA/ESA’s Hubble Space Telescope began observing the brilliant brightness of the supernova in February 2018 as part of the research program led by lead researcher and Nobel Laureate Adam Riess of the Space Telescope Science Institute (STScI) and Johns Hopkins University, in Baltimore, USA. The Hubble images center on the barred spiral galaxy NGC 2525, which is located in the constellation of Puppis in the Southern Hemisphere.

The supernova is captured by Hubble in exquisite detail within this galaxy in the left portion of the image. It appears as a very bright star located on the outer edge of one of its beautiful swirling spiral arms. This new and unique time-lapse of Hubble images created by the ESA/Hubble team shows the once bright supernova initially outshining the brightest stars in the galaxy, before fading into obscurity during the year of observations. This time-lapse consists of observations taken over the course of one year, from February 2018 to February 2019.

“No Earthly fireworks display can compete with this supernova, captured in its fading glory by the Hubble Space Telescope,” shared Riess of this new time-lapse of the supernova explosion in NGC 2525.

Supernovae are powerful explosions which mark the end of a star’s life. The type of supernova seen in these images, known as a Type Ia supernova, originate from a white dwarf in a close binary system accreting material from its companion star. If the white dwarf reaches a critical mass (1.44 times the mass of our Sun), its core becomes hot enough to ignite carbon fusion, triggering a thermonuclear runaway process that fuses large amounts of oxygen and carbon together in a matter of seconds. The energy released tears the star apart in a violent explosion, ejecting matter at speeds up to 6% the speed of light and emitting huge amounts of radiation. Type Ia supernovae consistently reach a peak brightness of 5 billion times brighter than our Sun before fading over time.

Because supernovae of this type produce this fixed brightness, they are useful tools for astronomers, known as ‘standard candles’, which act as cosmic tape measures. Knowing the actual brightness of the supernova and observing its apparent brightness in the sky, astronomers can calculate the distance to these grand spectacles and therefore their galaxies. Riess and his team combined the distance measurements from the supernovae with distances calculated using variable stars known as Cepheid variables. Cepheid variables pulsate in size, causing periodic changes in brightness. As this period is directly related to the star’s brightness, astronomers can calculate the distance to them: allowing them to act as another standard candle in the cosmic distance ladder.

Riess and his team are interested in accurately measuring the distance to these galaxies since it helps them better constrain the expansion rate of the Universe, known as the Hubble constant. This value accounts for how fast the Universe is expanding depending on its distance from us, with more distant galaxies moving faster away from us. Since it launched, NASA/ESA’s Hubble Space Telescope has helped dramatically improve the precision of the Hubble constant. Results from the same observing program led by Riess have now reduced the uncertainty of their measurement of the Hubble constant to an unprecedented 1.9%. Further measurements of NGC 2525 will contribute to their goal of reducing the uncertainty down to 1%, pinpointing how fast the Universe is expanding. A more accurate Hubble constant may uncover clues about the invisible dark matter and mysterious dark energy, responsible for accelerating the Universe’s rate of expansion. Together this information can help us understand the history and future fate of our Universe.

A supermassive black hole is also known to be lurking at the centre of NGC 2525. Nearly every galaxy contains a supermassive black hole, which can range in mass from hundreds of thousands to billions of times the mass of the Sun.

Provided by ESA/Hubble Information Center

The Universe Is Expanding, But How Fast Is Up For Debate (Physics)

In the 1920s, astronomer Edwin Hubble brought about the modern age of cosmology when he discovered that the universe is expanding at a predictable rate, which has since been called the Hubble constant. Nearly 100 years later, more precise measurements have sharpened his accuracy — but may also put our current understanding of physics in limbo.

At the turn of the 20th century, the hot topic in cosmological circles was nebulae. We could see them with telescopes, but we didn’t know how far away they were — were they interstellar clouds within the Milky Way, or were they far-off galaxies of their own? If you’ve ever wondered whether a light in the night sky is a satellite or a star, you know how difficult it is to tell the distance of things in space. In the early 1900s, Henrietta Swan Leavitt discovered what became known as Cepheid variables, a type of star whose brightness varied at a rate that could be used to calculate their absolute luminosity, or intrinsic brightness. That’s important because astronomers can use a star’s luminosity to measure its distance from us. A decade or so later, Edwin Hubble realized that many nebulae contained these variable stars, and could, therefore, make the important discovery that nebulae weren’t located in our own galaxy, but far beyond it, existing as galaxies in their own right.

Oh, but that’s not all. Next, Hubble compared these distance measurements with each galaxy’s velocity and found that the further away the galaxy was, the faster it was moving away from us. That led to the bombshell of the century: the universe was expanding. (To understand why more distant galaxies are moving faster, imagine a loaf of raisin-bread dough. When you put it in the oven, all of the raisins are evenly distributed, but as it rises in the oven, the raisins near the edges move outward faster than those in the center.) Hubble’s formula to determine the speed of a galaxy, called Hubble’s law, is v = H0d. H0 is, you guessed it, the Hubble constant, which astronomers have used ever since to judge the rate at which the universe is expanding.

When Hubble made this discovery, technology was not what it is today, to put it mildly. As a result, Hubble’s estimate for the value of the Hubble constant was pretty imprecise. One big reason for launching the Hubble Space Telescope in the 1980s was to get a more precise estimate for a number that at that time was somewhere between 50 and 100 km/sec/Mpc (kilometers per second per Megaparsec) — they wanted to whittle the accuracy down to at least 10 percent, which is still a wildly imprecise margin for science. Fast forward 30 years, and even more precise instruments such as the Wilkinson Microwave Anisotropy Probe (WMAP) honed the number to 69.3 km/sec/Mpc. Then in 2013, the expansion rate of the universe put on the brakes when the Planck satellite used background radiation from the Big Bang to find that the Hubble constant was closer to 67 km/sec/Mpc.

But in December 2016, a group called H0 Lenses in COSMOGRAIL’s Wellspring, or H0liCOW (get it?), used Einstein’s theory of general relativity to determine that the Hubble constant was a much faster 72 km/sec/Mpc. Despite its name, improved technology means that the Hubble constant keeps changing. What does that mean? A lot. It could mean that there are yet discovered elementary particles at play. It could mean that dark energy, which was previously blamed for shifts in the expansion rate, isn’t there at all, and is instead a theoretical form called phantom energy. This all would mean new physics and a drastic change in our understanding of the universe. But for now? It’s too soon to tell.

Time Maybe Slowing Down – And Will Eventually Stop (Physics)

The universe is expanding at an ever-accelerating rate. At least, that’s what the vast majority of scientists would have you believe. But according to a team of Spanish physicists, it may not be the expansion of the universe that’s changing rate, but time itself. Time might be slowing down, and that means that it could eventually stop altogether.

To illustrate what José Senovilla and his team at the University of the Basque Country in Bilbao, Spain are getting at, think about what it sounds like when an ambulance passes you on the street, sirens blazing. As it drives away from you, the siren begins to drop in pitch. This is known as the Doppler effect, and it happens because the sound waves ever so slightly stretch as the ambulance drives away from you, meaning they reach you at a slower rate (i.e. a lower frequency).

But what if the laws of physics changed when that ambulance passed, and instead of its speed causing that drop in frequency, it was the passage of time? If time were slowing down, that would also make the sound waves reach you at a lower frequency. That’s essentially what Senovilla’s team is suggesting. We “know” the universe is expanding at an accelerating rate because galaxies further away from us have a greater redshift — light’s version of that ambulance Doppler effect — than galaxies closer to us, meaning they’re moving faster. But if time were slowing down, the light would just reach us at a lower frequency. We’d see the redshift, but it would be for a different reason.

This theory sounds outlandish, but it fixes some nagging problems. For the universe’s expansion to be accelerating, you need to come up with something to cause it. That’s where so-called “dark energy” comes in. This mysterious force is supposed to make up 68 percent of the universe, but we’ve never actually observed it. If time is slowing down instead, you don’t need dark energy at all. The mystery of dark energy is fixed since it never existed in the first place.

This theory sounds outlandish, but it fixes some nagging problems. For the universe’s expansion to be accelerating, you need to come up with something to cause it. That’s where so-called “dark energy” comes in. This mysterious force is supposed to make up 68 percent of the universe, but we’ve never actually observed it. If time is slowing down instead, you don’t need dark energy at all. The mystery of dark energy is fixed since it never existed in the first place.

But this theory gets weirder. That’s because it’s based on a principle in string theory that says our universe exists on the surface of a membrane — a “brane,” in string-theory speak — that itself exists inside a higher-dimensional space called the “bulk,” aka hyperspace. All branes can have different numbers of dimensions; ours happens to have three spatial dimensions and one time dimension, but others could have no time dimensions or multiple time dimensions. Dimensions in those other branes could even swing between different versions: space could become time and vice versa. That’s what the researchers think might be happening to our time dimension: It’s slowly turning into a space dimension. If it succeeded, our universe would be frozen in time and exist in four-dimensional space.

We’d experience this as a gradual slowing of time — so gradual, in fact, that for the first billion years or so, we’d only see its evidence in grand scales, like the movement of faraway galaxies. “Our calculations show that we would think that the expansion of the universe is accelerating,” Senovilla told New Scientist. “[Any] observation of dark energy could be evidence that our brane is changing signature and that time is disappearing.”

But if this sounds alarming, don’t worry: This won’t happen for billions of years. In the meantime, buck up! Life is longer than you thought.