June 15, 2010
Is the Universe Stranger Than We Know? What if Dark Energy Does Not Exist?
It is the pattern of ripples detected by microwave background telescopes such as WMAP that underpin the idea that the Universe is composed of 22% dark exotic particles and 74% dark energy with the remaining 4% being the atoms in the ordinary material that we see around us.
This model produces a largest ripple size of about 1 degree on the microwave sky and this is well matched by the ripples seen in the WMAP data. So these WMAP ripples have a size that is roughly twice the size of the Full Moon as they appear on the sky. Models that don’t have dark energy or dark matter tend to produce CMB ripples that are smaller, only about half the standard model size and so just about the size of the Full Moon.
The radio sources have the advantage that they are much closer in brightness to the CMB ripples that are being studied than Jupiter which is ~1000 times brighter. But their faintness is also a disadvantage which means that the Durham team have had to stack hundreds of the radio sources to get their result.
If the WMAP CMB map is smoothed by as much as the radio sources appear to be then it may make it more easy for other models without dark matter (or dark energy!) to fit the CMB data. It will then be interesting to see if the new European PLANCK satellite, currently taking data, will confirm the WMAP results. The PLANCK telescope will also smooth the new CMB maps and again the radio source technique used by Sawangwit and Shanks can be used to help them judge how much.
The same Durham team were also involved with international collaborators in another recent paper which suggested that an independent CMB check on the existence of dark energy might not be as “bullet-proof “ as previously thought.
If dark energy exists it causes the expansion of the Universe to accelerate at late times. CMB photons have to pass through giant superclusters of galaxies on their way to be detected by telescopes such as WMAP. Normally a CMB photon gets gravitationally blueshifted as it enters a cluster and redshifted as it leaves and the two effects cancel.
But if the cluster galaxies accelerate away from each other as the photon passes through then the cancellation is not exact and a trace is left in that slightly higher CMB temperatures should be observed in sightlines that pass near to galaxy superclusters.
Previously claims have been made that this “ISW “ signal is seen at high significance when CMB-galaxy correlations are studied. But in a powerful new sample of ~1 million luminous red galaxies from the Sloan Digital Sky Survey no such effect is seen and when this result is included, the significances of the previous detections reduce to the point where they are as consistent with a zero detection of dark energy as with the standard model prediction.
If the same null result is seen in the Southern Hemisphere using WMAP and PLANCK CMB data coupled with millions of galaxies to be found in new Southern Surveys such as the ESO VST ATLAS (PI T. Shanks) then again there will be a significant threat to the standard cosmological model in which dark energy plays a vital role.
The odds are still that the standard model with its dark energy and dark matter will survive but there are certainly many theorists who might hope that it does not! The identification of dark matter with exotic particles as yet undetected in the laboratory and the introduction of dark vacuum energy in an amount that is minute compared to the total energy of the Universe at early-times leaves many cosmologists feeling unsure.
The dark energy problem is particularly severe – most theorists would prefer a zero cosmological constant because it might be hoped that it could be explained by some as yet unknown symmetry of nature. Indeed, if there had to be a cosmological constant then the string theorists of particle physics would actually prefer that it was negative which is the opposite to what is apparently observed in the supernova Hubble diagram. These problems frequently cause theorists to resort to the “anthropic principle” for an explanation.
The standard model also has astrophysical difficulties. For example, in galaxy formation theories, as much “feedback” energy is now being used in preventing stars from forming as in forming them under gravity, seemingly at odds with the simplest “bottom-up” picture of galaxy formation.
Even the evidence for dark matter is less strong than it was in the 1930’s when Fritz Zwicky first discovered the “missing mass” problem in the centres of rich galaxy clusters. The confirmation from X-ray satellites like Chandra and XMM-Newton that these galaxy clusters contain large amounts of hot gas as well as galaxies and stars has reduced the missing mass/dark matter discrepancy by a factor of 10-100! It remains to be seen whether the remaining factor or 4-5 merits the invoking of a cosmological density of exotic particles as required by the standard model.
The undoubted successes of the standard cosmological model therefore have to be balanced against the above problems. Much depends on the results from the “precision” Cosmic Microwave Background experiments. If these are correct then the standard model, with all its difficulties, will likely be correct. This is why tests of the CMB results such as those made by the Durham team and their collaborators are so important for cosmology.
The effect of the WMAP telescope on the CMB ripples and the search for the signature of dark energy in the CMB-galaxy correlations are crucial for the survival of the standard model. The results at the least give the CMB observational teams a chance to check whether their systematic errors are really well enough established to reject all simpler cosmological models and only accept the standard model with its mysterious dark matter and dark energy components.
The WMAP team, according to New Scientist isn't taking the challenge lighly. They claim that the radio sources observed by WMAP coincide with spots of the sky where the temperature is slightly higher, making the calibration inaccurate. "We're happy to defend WMAP," says team member Gary Hinshaw of NASA Goddard Space Flight Center in Greenbelt, Maryland.
According to their critics, to explain away theses potentially undermining errors, standard model and WMAP supporters have invented "dark energy" and "great attractors" so as to explain why a created universe did not spread out uniformly at the same speed and in the same spoke-like directions as predicted by theory.
Predications based on the Big Bang can account for less than 20% of the mass and density of the known, observable Hubble length universe. Nor can this theory explain gravity, the discordant data on red shifts, galaxy distribution, colliding galaxies, the abundance of hydrogen and helium, the existence of elementary particles, and why the movement of distant galaxies appears to be accelerating.
Critics of the standard model say that only the addition of ad hoc hypothetical appendages and parameters which are constantly adjusted have prevented the Big Bang theory from complete collapse.
Casey Kazan via University of Durham
astro.dur.ac.uk/~dph3us/cmb_press_release.doc
No comments:
Post a Comment