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Say goodbye to dark matter and energy?
KEITH COOPER
ASTRONOMY NOW
Posted: 15 June 2010


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The idea of a Universe dominated by dark energy and dark matter has been thrown into jeopardy by a reassessment of observations of the cosmic microwave background (CMB) radiation.

Dark energy and dark matter have become cornerstones of the standard model of cosmology, despite nobody knowing exactly what they are. Dark energy is apparently causing the expansion of the Universe to accelerate, whilst dark matter has to be inferred to explain unexplained gravitational effects. Both have several lines of evidence backing them, but one of the most crucial is the observations of the CMB radiation, which is radiation emitted just 380,000 after the big bang.

The map of the CMB after seven years of gathering data by WMAP. Image: NASA/WMAP Science Team.

The early Universe was filled with a sea of plasma – in other words ionised atoms and swarms of free electrons that would scatter photons of light. Acoustic vibrations oscillated through this plasma, but after 380,000 years the Universe cooled sufficiently for the ions to soak up all the free electrons and allow photons of light to continue into the Universe unhindered. These photons have today been stretched into microwave wavelengths, but they still carry the pattern of the vibrations, indicating regions of the Universe where matter was more or less dense. However, graduate student Utane Sawangwit and Professor Tom Shanks from the University of Durham have published a paper in the Monthly Notices of the Royal Astronomical Society that describes how the way WMAP has been measuring the size of these ripples may be wrong. Consequently, because the size of the ripples depends on the composition of the Universe, this puts dark matter and dark energy – which supposedly make up 96 percent of the matter and energy in the Universe – into doubt.

The heart of the problem is found with the WMAP spacecraft itself, which officially has a ‘beam width’ of 12 arcminutes, but Sawangwit and Shanks have found that in fact it smears its observations by several degrees, which has the effect of making the ripples look bigger than they really are. To double check, they independently tested how large distant astrophysical radio objects appeared to WMAP compared to how they appear in radio telescopes, and saw the same smearing effect.

“It is still unanswered why the radio sources taken from the WMAP observations give a different profile to the ones the WMAP team are using to do their cosmology,” Shanks tells Astronomy Now. The WMAP science team have argued that what the Durham astronomers are seeing are statistical fluctuations called ‘Eddington bias’, but Shanks and Sawangwit respond by saying that independent checks of their radio sources by other surveys operating at different frequencies than WMAP also give the same results for the size of the radio objects.

According to the standard model of the Universe dark matter makes up 23 percent of the Universe, dark energy 73 percent and normal matter just four percent. Image: NASA/WMAP Science Team.

If the ripples are in fact smaller than we have realised, then it affects not only dark matter and dark energy, but also what we understand about the entire evolution of the Universe and the growth of galaxies.

“How sound waves oscillated in the early Universe depended on the composition of the Universe, and it turns out that the ripples would be larger with cold dark matter than without,” says Shanks. The fact that his results indicate the ripples are smaller therefore puts a question mark over exotic dark matter, although he himself admits that there is other evidence for some form of dark matter.

“I’m not saying there isn’t dark matter, what I’m querying is the evidence for dark matter made up of exotic particles,” he says. “When Fritz Zwicky first detected the missing mass problem in the 1930s the ratio between the matter that he could see and that which he couldn’t but was needed to keep a galaxy cluster stable was about 600. Today X-ray satellites have shown there are huge amounts of hot gas in these clusters so now we’re only looking for a factor of four or five in terms of what’s missing. Of course a factor of four or five is still a significant discrepancy, but is it worth invoking particles that haven’t been discovered yet? I sometimes wonder if Zwicky was still alive today whether he would still think there was a problem.”

Dark energy is also targeted by the Durham astronomers. As CMB photons travel through superclusters of galaxies they first become blueshifted by the cluster’s gravity, and upon leaving the cluster become redshifted again. The blueshift and redshift would cancel each other out were it not for the fact that dark energy is expanding space and causing galaxies to move away from each other, resulting in the CMB photons becoming slightly blueshifted overall. However, a search for this blueshifting by Sawangwit and Shanks with the Sloan Digital Sky Survey has failed to turn up evidence for it.

“The odds are that the standard model will still survive because there is evidence for, but the CMB has a huge influence on cosmological models,” says Shanks. “The conclusion of our results is that there is much more flexibility in the CMB for models other than ones with dark energy and exotic dark matter.”

Find out more about the CMB and WMAP at http://map.gsfc.nasa.gov/.

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