Decoding the gravitational evolution of dark matter halos

Researchers at Kavli IPMU and their collaborators have revealed that considering environmental effects such as a gravitational tidal force spread over a scale much larger than a galaxy cluster is indispensable to explain the distribution and evolution of dark matter halos around galaxies. A detailed comparison between theory and simulations made this work possible. The results of this study, which are published in Physical Review D as an Editors' Suggestion, contribute to a better understanding of fundamental physics of the universe.

In the standard scenario for the formation of a cosmic structure, dark matter, which has an energy budget in the universe that is approximately five times greater than ordinary matter (e.g., atoms), first gathers gravitationally to form a crowded region, the so-called dark matter halos. Then these dark matter halos attract atomic gas and eventually form stars and galaxies. Hence, to extract cosmological information from a three-dimensional galaxy map observed in SDSS BOSS, the SuMIRe project, etc., it is important to understand how clustering of dark matter halos has gravitationally evolved throughout cosmic history. (This is referred to as the halo bias problem.)
"Various studies have described the halo bias theoretically," said Teppei Okumura, a project researcher involved in the study from Kavli IPMU. "However, none of them reproduced simulation results well. So, we extended prior studies motivated by a mathematical symmetry argument and examined if our extension works."
The authors demonstrate that higher-order nonlocal terms originating from environmental effects such as gravitational tidal force must be taken into account to explain the halo bias in simulations. They also confirm that the size of the effect agrees well with a simple theoretical prediction.
"The results of our study allow the distribution of dark matter halos to be more accurately predicted by properly taking into account higher-order terms missed in the literature," said Shun Saito, the principal investigator of the study from Kavli IPMU. "Our refined model has been already applied to actual data analysis in the BOSS project. This study certainly improves the measurement of the nature of dark energy or neutrino masses. Hence, it has led to a better understanding of the fundamental physics of the universe."
This study is supported by a Grant-in-Aid from the Japan Society for the Promotion of Science (JSPS) No. 25887012.

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The above story is based on materials provided by Kavli Institute for the Physics and Mathematics of the Universe. Note: Materials may be edited for content and length.

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Journal Reference:

1. Shun Saito, Tobias Baldauf, Zvonimir Vlah, Uroš Seljak, Teppei Okumura, Patrick McDonald. Understanding higher-order nonlocal halo bias at large scales by combining the power spectrum with the bispectrum. Physical Review D, 2014; 90 (12) DOI: 10.1103/PhysRevD.90.123522

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Dynamic, dark energy in an accelerating universe | sci-english.blogspot.com

Irene Sendra-Server, PhD holder in Physics and research scientist in the UPV/EHU’s Department of Theoretical Physics and History of Science | sci-english.blogspot.com

The models proposed by the UPV/EHU-University of the Basque Country researcher are contributing towards understanding the nature of dark energy.

It was cosmology that drew Irene Sendra from Valencia to the Basque Country. Cosmology also gave her the chance to collaborate with one of the winners of the 2011 Nobel Prize for Physics on one of the darkest areas of the universe. And dark matter and dark energy, well-known precisely because so little is known about them, are in fact the object of the study by Sendra, a researcher in the Department of Theoretical Physics and History of Science of the UPV/EHU's Faculty of Science and Technology.
"Observations tell us that about 5% of the universe is made up of ordinary matter; 22% corresponds to dark matter, which we know exists because it interacts gravitationally with ordinary matter; another 73% is dark energy, which is known to be there because otherwise one would not be able to account for the accelerating expansion of the universe," explains Irene Sendra; "We are trying to find out a bit more about what dark energy is," she adds.
If dark energy did not exist, the gravitational pull exerted by matter would slow down the expansion of the universe, but observations have concluded that the opposite is the case.Dark energy is what makes the universe expand in an accelerating way, and contributing towards understanding its nature is the basis of the research Sendra has done as part of her PhD thesis entitled: "Cosmology in an accelerating universe: observations and phenomenology."
The research starts with the hypothesis that dark energy could be dynamic.The most widely accepted model, known as the Lambda-CDM, explains the acceleration of the universe by means of the cosmological constant, whose equation of state would have a value of -1, constant throughout the whole evolution of the universe.However, there are observations which this model cannot account for."We look for a dynamic, dark energy that would vary over time; we apply various models to the observable data, we play around with small disturbances, and we see whether they adapt better than a constant," explains Sendra.
Making use of mathematical and statistical tools, the values that the observation proposes for the parameters studied are compared with those proposed by the model."So,throughmany iterations, we can see which values would take the constants of our model.The equation of state of dark energy is worth practically -1 now, but it appears to have evolved from different values in the past; however, there is still a high percentage of error in determining these values."According to Sendra's calculations, these data are consistent with dynamic dark energy, which would vary with the redshift observed in the universe.Results that have yet to be published and obtained in collaboration with Adam Riess, the 2011 Nobel Prize Winner for Physics, go further in that direction.
In this PhD thesis, besides studying the equation of state of dark energy, a new model has been proposed and it would unite dark energy with dark matter.As Sendra explains, "They could be the same thing that is manifested in a different way depending on the context; we have explained the effect of both of them through one single component, and the observations give better results in this model than in others that try to unite matter and dark energy."
Finally, Sendra has peered at the oldest universe by means of the study of its cosmic microwave background."It is the most distant proof we have of the universe," she comments, "and the study of it tells us that the actual number of neutrinos is higher than three.Nevertheless, what we know for a fact, through the standard model of particles, is that there are three kinds of neutrinos.We have ended up with a somewhat strange value, so we are trying to account for that excess in the number of neutrinos." Sendra's proposal is heading in the direction of the string theory. According to her results, this neutrino excess could be interpreted as the contribution of primordial gravitational waves, caused by the interaction of cosmic strings at the time when the cosmic microwave background was produced.

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The above story is based on materials provided by Basque Research. Note: Materials may be edited for content and length.

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Dark energy is real, say astronomers | sci-english.blogspot.com

A visual impression of the data used in the study. The relevant extra-galactic maps are represented as shells of increasing distance from Earth | sci-english.blogspot.com

Dark energy, a mysterious substance thought to be speeding up the expansion of the Universe is really there, according to a team of astronomers at the University of Portsmouth and LMU University Munich. After a two-year study led by Tommaso Giannantonio and Robert Crittenden, scientists conclude that the likelihood of its existence stands at 99.996 per cent. Their findings are published in the Monthly Notices of the Royal Astronomical Society.

Professor Bob Nichol, a member of the Portsmouth team, said: "Dark energy is one of the great scientific mysteries of our time, so it isn't surprising that so many researchers question its existence.
"But with our new work we're more confident than ever that this exotic component of the Universe is real -- even if we still have no idea what it consists of."
Over a decade ago, astronomers observing the brightness of distant supernovae realised that the expansion of the Universe appeared to be accelerating. The acceleration is attributed to the repulsive force associated with dark energy now thought to make up 73 per cent of the content of the cosmos. The researchers who made this discovery received the Nobel Prize for Physics in 2011, but the existence of dark energy remains a topic of hot debate.
Many other techniques have been used to confirm the reality of dark energy but they are either indirect probes of the accelerating Universe or susceptible to their own uncertainties. Clear evidence for dark energy comes from the Integrated Sachs Wolfe effect named after Rainer Sachs and Arthur Wolfe.
The Cosmic Microwave Background, the radiation of the residual heat of the Big Bang, is seen all over the sky. In 1967 Sachs and Wolfe proposed that light from this radiation would become slightly bluer as it passed through the gravitational fields of lumps of matter, an effect known as gravitational redshift.
In 1996, Robert Crittenden and Neil Turok, now at the Perimeter Institute in Canada, took this idea to the next level, suggesting that astronomers could look for these small changes in the energy of the light, or photons, by comparing the temperature of the radiation with maps of galaxies in the local Universe.
In the absence of dark energy, or a large curvature in the Universe, there would be no correspondence between these two maps (the distant cosmic microwave background and relatively closer distribution of galaxies), but the existence of dark energy would lead to the strange, counter-intuitive effect where the cosmic microwave background photons would gain energy as they travelled through large lumps of mass.
The Integrated Sachs Wolfe effect was first detected in 2003 and was immediately seen as corroborative evidence for dark energy, featuring in the 'Discovery of the year' in Science magazine. But the signal is weak as the expected correlation between maps is small and so some scientists suggested it was caused by other sources such as the dust in our galaxy. Since the first Integrated Sachs Wolfe papers, several astronomers have questioned the original detections of the effect and thus called some of the strongest evidence yet for dark energy into question.
In the new paper, the product of nearly two years of work, the team have re-examined all the arguments against the Integrated Sachs Wolfe detection as well as improving the maps used in the original work. In their painstaking analysis, they conclude that there is a 99.996 per cent chance that dark energy is responsible for the hotter parts of the cosmic microwave background maps (or the same level of significance as the recent discovery of the Higgs boson).
"This work also tells us about possible modifications to Einstein's theory of General Relativity," notes Tommaso Giannantonio, lead author of the present study.
"The next generation of cosmic microwave background and galaxy surveys should provide the definitive measurement, either confirming general relativity, including dark energy, or even more intriguingly, demanding a completely new understanding of how gravity works."

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The above story is based on materials provided by Royal Astronomical Society (RAS). Note: Materials may be edited for content and length.

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Journal Reference:

1. T. Ginnantonio, R. Crittenden, R. Nichol, A. Ross. The significance of the integrated Sachs-Wolfe effect revisited. Monthly Notices of the Royal Astronomical Society, 2012; (in press) [link]

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Best way to measure dark energy just got better | sci-english.blogspot.com

A Type Ia supernova occurs when a white dwarf accretes material from a companion star until it exceeds the Chandrasekhar limit and explodes. By studying these exploding stars, astronomers can measure dark energy and the expansion of the universe. CfA scientists have found a way to correct for small variations in the appearance of these supernovae, so that they become even better standard candles. The key is to sort the supernovae based on their color | sci-english.blogspot.com

Dark energy is a mysterious force that pervades all space, acting as a "push" to accelerate the Universe's expansion. Despite being 70 percent of the Universe, dark energy was only discovered in 1998 by two teams observing Type Ia supernovae. A Type 1a supernova is a cataclysmic explosion of a white dwarf star.

These supernovae are currently the best way to measure dark energy because they are visible across intergalactic space. Also, they can function as "standard candles" in distant galaxies since the intrinsic brightness is known. Just as drivers estimate the distance to oncoming cars at night from the brightness of their headlights, measuring the apparent brightness of a supernova yields its distance (fainter is farther). Measuring distances tracks the effect of dark energy on the expansion of the Universe.
The best way of measuring dark energy just got better, thanks to a new study of Type Ia supernovae led by Ryan Foley of the Harvard-Smithsonian Center for Astrophysics. He has found a way to correct for small variations in the appearance of these supernovae, so that they become even better standard candles. The key is to sort the supernovae based on their color.
"Dark energy is the biggest mystery in physics and astronomy today. Now, we have a better way to tackle it," said Foley, who is a Clay Fellow at the Center. He presented his findings in a press conference at the 217th meeting of the American Astronomical Society.
The new tool also will help astronomers to firm up the cosmic distance scale by providing more accurate distances to faraway galaxies.
Type Ia supernovae are used as standard candles, meaning they have a known intrinsic brightness. However, they're not all equally bright. Astronomers have to correct for certain variations. In particular, there is a known correlation between how quickly the supernova brightens and dims (its light curve) and the intrinsic peak brightness.
Even when astronomers correct for this effect, their measurements still show some scatter, which leads to inaccuracies when calculating distances and therefore the effects of dark energy. Studies looking for ways to make more accurate corrections have had limited success until now.
"We've been looking for this sort of 'second-order effect' for nearly two decades," said Foley.
Foley discovered that after correcting for how quickly Type Ia supernovae faded, they show a distinct relationship between the speed of their ejected material and their color: the faster ones are slightly redder and the slower ones are bluer.
Previously, astronomers assumed that redder explosions only appeared that way because of intervening dust, which would also dim the explosion and make it appear farther than it was. Trying to correct for this, they would incorrectly calculate that the explosion was closer than it appeared. Foley's work shows that some of the color difference is intrinsic to the supernova itself.
The new study succeeded for two reasons. First, it used a large sample of more than 100 supernovae. More importantly, it went back to "first principles" and reexamined the assumption that Type Ia supernovae are one average color.
The discovery provides a better physical understanding of Type Ia supernovae and their intrinsic differences. It also will allow cosmologists to improve their data analysis and make better measurements of dark energy -- an important step on the road to learning what this mysterious force truly is, and what it means for the future of the cosmos.

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The above story is based on materials provided by Harvard-Smithsonian Center for Astrophysics. Note: Materials may be edited for content and length.

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