Archive for ‘Dark energy’

October 6, 2011

Scientists release most accurate simulation of the universe to date

The Bolshoi (Большой) supercomputer simulation is a simulation of a substantial portion (a cube one billion light-years on a side) of the entire universe. Of course, any simulation is limited in the level of detail (the resolution) it can handle, which depends on the computing power used to run the model. A simulation also relies on the most accurately measured values available of a number of parameters. This new Bolshoi simulation has almost ten times the resolution of the best previous simulation. It also uses an updated set of parameters, replacing the previous set, which wasn’t as accurate.

Scientists release most accurate simulation of the universe to date – UC Santa Cruz

The Bolshoi supercomputer simulation, the most accurate and detailed large cosmological simulation run to date, gives physicists and astronomers a powerful new tool for understanding such cosmic mysteries as galaxy formation, dark matter, and dark energy.

The simulation traces the evolution of the large-scale structure of the universe, including the evolution and distribution of the dark matter halos in which galaxies coalesced and grew. Initial studies show good agreement between the simulation’s predictions and astronomers’ observations.

So what does a sophisticated simulation of this kind actually tell us about the history of the universe, including parts we can’t observe directly?

October 5, 2011

Galaxy Clusters Validate Einstein’s Theory – and Dark Matter Too

The theory of relativity – both special relativity and general relativity – has now been tested successfully so many ways ever since its early days that further tests seem almost redundant. (That little mix-up with superluminal neutrinos to the contrary notwithstanding – since it’s at most a higher order correction, but more likely an error in data interpretation.) But science never stops testing if there’s some creative new way to do so. And that’s what’s involved here.

One of the predictions of general relativity is that light photons lose energy when climbing out of a strong gravitational field. In that case, the light should be redshifted, in a way that doesn’t involve either a Doppler effect or the expansion of the universe.

It takes a very strong gravitational field to make this gravitational redshift apparent. Even a single galaxy isn’t large enough. But a cluster of galaxies will do, although the effect is almost masked by relative motion of galaxies in the cluster. However, in a cluster there’s a fairly straightforward way to check, since the shift should be larger with photons from the center of the cluster than from the periphery, as the gravitational field is stronger at the center.

Galaxy Clusters Validate Einstein’s Theory – ScienceNOW

Wojtak and his colleagues knew that measuring gravitational redshifting within a single galaxy cluster would be difficult because the effect is very small and needs to be teased apart from the redshifting caused by the orbital velocity of individual galaxies within the cluster and the redshifting caused by the expansion of the universe. The researchers approached the problem by averaging data collected from 8000 galaxy clusters by the Sloan Digital Sky Survey. The hope was to detect gravitational redshift “by studying the properties of the redshift distribution of galaxies in clusters rather than by looking at redshifts of individual galaxies separately,” Wojtak explains.

Sure enough, the researchers found that the light from the clusters was redshifted in proportion to the distance from the center of the cluster, as predicted by general relativity. “We could measure small differences in the redshift of the galaxies and see that the light from galaxies in the middle of a cluster had to ‘crawl’ out through the gravitational field, while it was easier for the light from the outlying galaxies to emerge,” Wojtak says.

In addition to further confirming relativity, this research may also rule out some alternative theories of gravity that have been proposed to avoid the hypotheses of dark matter and dark energy, which are needed with standard relativity to explain many astronomical observations – and do so very well.

September 21, 2011

Gamma-ray bursts shed light on the nature of dark energy

Dark energy was “discovered” unexpectedly in 1997 (published in 1998), when a survey of relatively nearby Type Ia supernovae found something strange. These supernovae should all have about the same intrinsic luminosity. But instead, using assumptions current at the time about the rate of expansion of the universe, the farther away these supernovae were the less their intrinsic luminosity would have to be.

If in fact these supernovae have pretty much the same intrinsic luminosity in all cases, then they are “standard candles”. Hence a simple application of the inverse square law (apparent luminosity is proportional to the inverse square of the distance) would determine how far away the objects should be. However, by another chain of reasoning, the distance can be calculated using “Hubble’s law” from the observed redshift in the objects’ spectra, because the law states that relative velocity (which is inferred from redshift) is proportional to distance, with a constant of proportionality called “Hubble’s constant”. (The law was named after Edwin Hubble, who first conceived it.)

The problem was that this prediction of distance was too small for the observed luminosity. Otherwise stated, if the remote supernovae were actually as close as the distances calculated by Hubble’s law, they could not be as luminous intrinsically as more nearby supernovae. However, this problem would go away if there were an error in calculating the correct distance from Hubble’s law – which could happen if something now called “dark energy” existed.

In order to verify this hypothesis of dark energy, there are several reasons astrophysicists want to have another type of standard candle besides Type Ia supernovae for gauging very large distances. Ideally another type of standard candle that doesn’t depend on the behavior of Type Ia supernovae and works out to much larger distances could be identified. Astrophysicists have suspected that another type of supernova, which is responsible for a much more energetic pulse of electromagnetic radiation – a gamma-ray burst – can fill the bill.

Research just announced has studied the properties of relatively nearby gamma-ray bursts and identified certain characteristics that allow predicting the intrinsic brightness of the burst. Comparing that brightness to what’s actually observed determines the distance of the event.

Gamma-ray bursts shed light on the nature of dark energy – University of Warsaw

Dark energy is the basic constituent of the Universe today, one that is responsible for its accelerated expansion. Although astronomers observe the cosmological effects of the impact of dark energy, they still do not know exactly what it is. A new method for measuring the largest distances in the Universe developed by scientists from the Faculty of Physics, University of Warsaw and the University of Naples Federico II helps solve the mystery. A key role is played by the most powerful cosmic explosions – gamma-ray bursts.

What is the nature of dark energy, a recently discovered dominant constituent of the Universe today? Is expansion-accelerating dark energy an intrinsic property of space-time itself or rather a field unknown to science? A new distance-measuring method developed by scientists from the Faculty of Physics, University of Warsaw (FUW) and the University of Naples Federico II can provide the answer. “We are able to determine the distance of an explosion on the basis of the properties of the radiation emitted during gamma-ray bursts. Given that some of these explosions are related to the most remote objects in space that we know about, we are able, for the first time, to assess the speed of space-time expansion even in the relatively early periods after the Big Bang,” says Prof. Marek Demiański (FUW). The method was used to verify models of the structure of the Universe containing dark energy.