Now We Know That Dark Matter Isn’t Primordial Black Holes

For over fifty years, scientists have theorized that roughly 85% of matter in the Universe’s is made up of a mysterious, invisible mass. Since then, multiple observation campaigns have indirectly witnessed the effects that this “Dark Matter” has on the Universe. Unfortunately, all attempts to detect it so far have failed, leading scientists to propose some very interesting theories about its nature.

One such theory was offered by the late and great Stephen Hawking, who proposed that the majority of dark matter may actually be primordial black holes (PBH) smaller than a tenth of a millimeter in diameter. But after putting this theory through its most rigorous test to date, an international team of scientists led from the Kavli Institute for the Physics and Mathematics of the Universe (IPMU) has confirmed that it is not.

The team was led by Hiroko Niikura, a PhD candidate student with the Kavli IPMU, and included researchers from Japan, India and the US. As they indicate in their paper, which recently appeared in the journal Nature Astronomy, the lack of results in dark matter research led them to consider Hawking’s theory, which he first suggested in 1974.

This illustration shows how gravitational lensing works. The gravity of a large galaxy cluster is so strong, it bends, brightens and distorts the light of distant galaxies behind it. Credit: NASA, ESA, L. Calcada

This theory posits that the majority of dark matter is made up of primordial black holes (PBH) formed shortly after the Big Bang. As Prof. Masahiro Takada, the principal investigator of the IMPU and a co-author on the paper, told Universe Today via email:

“What Prof. Hawking predicted was that, if there were little patches of overdensity in the early Universe, such patches could create black holes… Once black holes are formed, they would behave like dark matter (because it is invisible and interacts with other particles only via gravity).”

This theory is attractive because it does not rely on the existence of any exotic (but as of yet, undiscovered) particles. What’s more, shortly after Hawking proposed this idea, astrophysicists discovered that cosmic inflation could generate patches of overdensity in the early Universe due to quantum fluctuations, which could have resulted in black holes.

The team tested this theory by using the Subaru Telescope at the Mauna Kea Observatory in Hawaii to observe the neighboring Andromeda Galaxy, which is located about 2.54 million light years away. Unlike most galaxies in our cosmic neighborhood, Andromeda is one of only 100 or so that is approaching our galaxy – at a rate of 110 km per second (68 mi per second) – and is destined to collide with it.

Andromeda Galaxy. Credit: Wikipedia Commons/Adam Evans

These and other factors contributed to it being the best candidates to test Hawking’s theory, explained Prof. Takada:

“The Andromeda galaxy is the largest, nearby galaxy containing many stars inside. For example, Andromeda is much bigger than the Magellan clouds that are dwarf galaxies. Hence, IF we can observe stars in Andromeda at one time, we could find microlensing events of a star, the flicker of its brightness, due to a foreground black holes that are passing in front of the star on the sky.”

If Hawking’s theory were in fact correct, the space between Andromeda and our galaxy would be filled with PBHs. This would result in a gravitational lensing effect, where the gravitational force of all these tiny black holes would cause the light rays coming from Andromeda’s stars to bend and become magnified.

This effect, which was first predicted by Einstein and his Theory of General Relativity in 1915, has been used many times by astronomers to view distant objects by taking advantage of the presence of massive objects in between them and Earth. However, the opportunities for such events are rare, requiring a fortuitous alignment between the observer, the distant object and the intervening one.

To maximize their chances of capturing an event, the researchers team used the Subaru Telescope’s Hyper Suprime-Cam digital camera, which is able to capture whole images of the Andromeda galaxy in a single shot. They also took multiple images of the galaxy to make sure that they caught any brief flickers coming from Andromeda’s stars.

Artist’s conception shows two merging black holes similar to those detected by LIGO on January 4th, 2017. Credit: LIGO/Caltech

These flickers would indicate that a primordial black hole was passing in front of them, thus distorting and magnifying their light. As Prof. Takada explained:

“If dark matter is PBH rather than elementary particles such as WIMPs, it could pass in front of a star in Andromeda galaxy and cause a microlensing event, the flicker of its brightness changing with observation time. This timescale in the change of star brightness depends on mass and velocity of PBH. If PBH is dark matter, we have a good understanding of its velocity: it should move with ~200km/s in interstellar space as the rotation curve of our Milky Way or Andromeda galaxy shows.”

All told, the team took 190 consecutive images of Andromeda over the course of seven hours and examined them closely for indications of a possible event. Given the expected mass of primordial black holes, at least 1000 events were anticipated. However, the team found evidence of only one event, which would indicate that primordial black holes would constitute less than 0.1% of dark matter mass.

That being said, this one possible event (which lasted for about an hour) was a significant discovery, since it is precisely what astronomers would expect for a light-mass PBH. As Takada indicated, this could be indirect evidence of a PBH caused by cosmic inflation. At the same time, it could be evidence of stellar variability (i.e. a stellar flare), so more observations are necessary before anything can be said definitively.

Illustris simulation, showing the distribution of dark matter in 350 million by 300,000 light years. Galaxies are shown as high-density white dots (left) and as normal, baryonic matter (right). Credit: Markus Haider/Illustris

Looking ahead, the team is planning on conducting further observations of the Andromeda galaxy to confirm their analysis. They also hope to investigate a new theory that posits how binary black holes – which have become detectable by LIGO thanks to the gravitational waves events they create – might in fact be primordial black holes.

“In brief, our results can’t entirely exclude the PBH-dark matter scenario, so dark matter could be an unknown elementary particle such as Weakly Interacting Massive Particle (WIMP),” concludes Prof. Takada. “In this case, we hope that underground experiments or accelerator experiments such as LHC will find such dark matter particles.”

In the meantime, the search for elusive dark matter continues! And much like the first-ever detection of gravitational waves, this discovery will trigger a revolution in the field of astronomy. And as Takada said, “it would be a Novel Prize discovery!”

Further Reading: Kavli IPMU, Nature Astronomy

Fuente: https://www.universetoday.com/141923/now-we-know-that-dark-matter-isnt-primordial-black-holes/

Searching for Dark Energy with the Whole World’s Supernova Dataset

Two views of one of the six new distant supernovae in the Supernova Cosmology Project's just-released Union2 survey, which among other refinements compares ground-based infrared observations (in this case by Japan's Subaru Telescope on Mauna Kea) with follow-up observations by the Hubble Space Telescope.

Narrower constraints from the newest analysis aren’t quite narrow enough.

The international Supernova Cosmology Project (SCP), based at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory, has announced the Union2 compilation of hundreds of Type Ia supernovae, the largest collection ever of high-quality data from numerous surveys. Analysis of the new compilation significantly narrows the possible values that dark energy might take—but not enough to decide among fundamentally different theories of its nature.

“We’ve used the world’s best-yet dataset of Type Ia supernovae to determine the world’s best-yet constraints on dark energy,” says Saul Perlmutter, leader of the SCP. “We’ve tightened in on dark energy out to redshifts of one”—when the universe was only about six billion years old, less than half its present age—“but while at lower redshifts the values are perfectly consistent with a cosmological constant, the most important questions remain.”

That’s because possible values of dark energy from supernovae data become increasingly uncertain at redshifts greater than one-half, the range where dark energy’s effects on the expansion of the universe are most apparent as we look farther back in time. Says Perlmutter of the widening error bars at higher redshifts, “Right now, you could drive a truck through them.”

As its name implies, the cosmological constant fills space with constant pressure, counteracting the mutual gravitational attraction of all the matter in the universe; it is often identified with the energy of the vacuum. If indeed dark energy turns out to be the cosmological constant, however, even more questions will arise.

“There is a huge discrepancy between the theoretical prediction for vacuum energy and what we measure as dark energy,” says Rahman Amanullah, who led SCP’s Union2 analysis; Amanullah is presently with the Oskar Klein Center at Stockholm University and was a postdoctoral fellow in Berkeley Lab’s Physics Division from 2006 to 2008. “If it turns out in the future that dark energy is consistent with a cosmological constant also at early times of the universe, it will be an enormous challenge to explain this at a fundamental theoretical level.”

A major group of competing theories posit a dynamical form of dark energy that varies in time. Choosing among theories means comparing what they predict about the dark energy equation of state, a value written w. While the new analysis has detected no change in w, there is much room for possibly significant differences in w with increasing redshift (written z).

“Most dark-energy theories are not far from the cosmological constant at z less than one,” Perlmutter says. “We’re looking for deviations in w at high z, but there the values are very poorly constrained.”

In their new analysis to be published in the Astrophysical Journal, “Spectra and HST light curves of six Type Ia supernovae at 0.511 < z < 1.12 and the Union2 compilation,” the Supernova Cosmology Project reports on the addition of several well-measured, very distant supernovae to the Union2 compilation. The paper is now available online at http://arxiv4.library.cornell.edu/abs/1004.1711.

Dark energy fills the universe, but what is it?

Dark energy was discovered in the late 1990s by the Supernova Cosmology Project and the competing High-Z Supernova Search Team, both using distant Type Ia supernovae as “standard candles” to measure the expansion history of the universe. To their surprise, both teams found that expansion is not slowing due to gravity but accelerating.

Other methods for measuring the history of cosmic expansion have been developed, including baryon acoustic oscillation and weak gravitational lensing, but supernovae remain the most advanced technique. Indeed, in the years since dark energy was discovered using only a few dozen Type Ia supernovae, many new searches have been mounted with ground-based telescopes and the Hubble Space Telescope; many hundreds of Type Ia’s have been discovered; techniques for measuring and comparing them have continually improved.

In 2008 the SCP, led by the work of team member Marek Kowalski of the Humboldt University of Berlin, created a way to cross-correlate and analyze datasets from different surveys made with different instruments, resulting in the SCP’s first Union compilation. In 2009 a number of new surveys were added.

The inclusion of six new high-redshift supernovae found by the SCP in 2001, including two with z greater than one, is the first in a series of very high-redshift additions to the Union2 compilation now being announced, and brings the current number of supernovae in the whole compilation to 557.

“Even with the world’s premier astronomical observatories, obtaining good quality, time-critical data of supernovae that are beyond a redshift of one is a difficult task,” says SCP member Chris Lidman of the Anglo-Australian Observatory near Sydney, a major contributor to the analysis. “It requires close collaboration between astronomers who are spread over several continents and several time zones. Good team work is essential.”

Union2 has not only added many new supernovae to the Union compilation but has refined the methods of analysis and in some cases improved the observations. The latest high-z supernovae in Union2 include the most distant supernovae for which ground-based near-infrared observations are available, a valuable opportunity to compare ground-based and Hubble Space Telescope observations of very distant supernovae.

Type Ia supernovae are the best standard candles ever found for measuring cosmic distances because the great majority are so bright and so similar in brightness. Light-curve fitting is the basic method for standardizing what variations in brightness remain: supernova light curves (their rising and falling brightness over time) are compared and uniformly adjusted to yield comparative intrinsic brightness. The light curves of all the hundreds of supernova in the Union2 collection have been consistently reanalyzed.

The upshot of these efforts is improved handling of systematic errors and improved constraints on the value of the dark energy equation of state with increasing redshift, although with greater uncertainty at very high redshifts. When combined with data from cosmic microwave background and baryon oscillation surveys, the “best fit cosmology” remains the so-called Lambda Cold Dark Matter model, or ΛCDM.

ΛCDM has become the standard model of our universe, which began with a big bang, underwent a brief period of inflation, and has continued to expand, although at first retarded by the mutual gravitational attraction of matter. As matter spread and grew less dense, dark energy overcame gravity, and expansion has been accelerating ever since.

To learn just what dark energy is, however, will first require scientists to capture many more supernovae at high redshifts and thoroughly study their light curves and spectra. This can’t be done with telescopes on the ground or even by heavily subscribed space telescopes. Learning the nature of what makes up three-quarters of the density of our universe will require a dedicated observatory in space.

This work was supported in part by the U.S. Department of Energy’s Office of Science.

Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research for DOE’s Office of Science and is managed by the University of California. Visit our website at http://www.lbl.gov.

The Supernova Cosmology Project’s Union2 compilation and reanalysis of decades of supernova surveys from the world’s leading researchers, with the addition of six high-redshift supernovae, puts new bounds on possible values for the nature of dark energy. Einstein’s cosmological constant comfortably fits the data, but there’s still plenty of room at the top for dynamical theories.

Fuente: newscenter.lbl.gov