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The expansion of the universe would be faster and faster

– News of February 27, 2018 –

The Hubble space telescope has been operational for almost 30 years and continues to deliver very important results. Recent observations have shown that the acceleration of the expansion of the universe is actually much faster than expected. The problem comes from a comparison of Hubble data with those of the Planck space observatory obtained a few years earlier. The measurements of both instruments are considered very reliable. They both sought to determine the Hubble constant, a parameter that describes the rate of expansion of the universe at a given moment. Planck’s data placed this constant between 67 km and 69 km per second per megaparsec. Planck had made these measurements by studying the cosmic microwave background, the very first light of the 13.8 billion-year-old universe.

New Hubble data estimates the Hubble constant at 63 km per second per megaparsec. Hubble has observed Cepheids, which are variable stars much more recent. Between the two measures, there is 9% difference. The problem is that there are difficulties in explaining it. The probability that an error has slipped into one of the two measures is incredibly low. These results seem to show that the expansion of the universe has accelerated in its history, and even faster than originally thought.

Among the possible explanations there is of course the dark energy. Failing to be well defined, it is a parameter that can be added to the standard model of cosmology to explain this type of observation. Another possibility is stronger interaction than expected between dark matter and more conventional materials. Finally new models continue to emerge to try to provide an explanation for this phenomenon, for example black radiation, sterile neutrinos that would be affected only by gravity.

The earliest evidence of accelerating universe expansion is very recent, dating back to the late 1990s, and it is likely that more decades of observation and speculation will be needed before consensus can be reached about the mechanism causing this acceleration. This is why we regret the very likely cancellation of WFIRST because the instrument would have allowed measurements of the Hubble constant at different ages of the universe. Fortunately, Europe has not canceled Euclid, its future space observatory dedicated to research on the expansion of the universe and dark energy. It should come into service at the beginning of the next decade. It will try to go back up to 10 billion years in the past. The different models of dark energy present tiny variations, so it will take measurements of very high precision to determine which track is the most interesting to follow.

Image by NASA / WMAP Science Team [Public domain], via Wikimedia Commons


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