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Universe is Expanding Uniformly, Scientists Say

The Universe is not spinning or stretched in any particular direction, according to a team of researchers from University College London and Imperial College London, UK.

A massive cluster of yellowish galaxies, seemingly caught in a red and blue spider web of eerily distorted background galaxies, makes for a spellbinding picture from Hubble. To make this unprecedented image of the cosmos, Hubble peered straight through the center of one of the most massive galaxy clusters known, called Abell 1689. The gravity of the cluster’s trillion stars - plus dark matter - acts as a 2-million-light-year-wide ‘lens’ in space. This gravitational lens bends and magnifies the light of the galaxies located far behind it. Some of the faintest objects in the picture are probably over 13 billion light-years away. Image credit: NASA / N. Benitez, JHU / T. Broadhurst, Racah Institute of Physics  Hebrew University / H. Ford, JHU / M. Clampin, STScI / G. Hartig, STScI / G. Illingworth, UCO  Lick Observatory / ACS Science Team / ESA.

A massive cluster of yellowish galaxies, seemingly caught in a red and blue spider web of eerily distorted background galaxies, makes for a spellbinding picture from Hubble. To make this unprecedented image of the cosmos, Hubble peered straight through the center of one of the most massive galaxy clusters known, called Abell 1689. The gravity of the cluster’s trillion stars – plus dark matter – acts as a 2-million-light-year-wide ‘lens’ in space. This gravitational lens bends and magnifies the light of the galaxies located far behind it. Some of the faintest objects in the picture are probably over 13 billion light-years away. Image credit: NASA / N. Benitez, JHU / T. Broadhurst, Racah Institute of Physics Hebrew University / H. Ford, JHU / M. Clampin, STScI / G. Hartig, STScI / G. Illingworth, UCO Lick Observatory / ACS Science Team / ESA.

Looking out into the night sky, we see a clumpy Universe: planets orbit stars in solar systems and stars are grouped into galaxies, which in turn form enormous galaxy clusters.

But cosmologists assume this effect is only local: that if we look on sufficiently large scales, the Universe is actually uniform.

The vast majority of calculations made about the Universe start with this assumption: that the Universe is broadly the same, whatever your position and in whichever direction you look.

If, however, the Universe was stretching preferentially in one direction, or spinning about an axis in a similar way to the Earth rotating, this fundamental assumption, and all the calculations that hinge on it, would be wrong.

Now, the research team led by University College London scientist Daniela Saadeh has put this assumption through its most stringent test yet and found only a 1 in 121,000 chance that the Universe is not the same in all directions.

“The finding is the best evidence yet that the Universe is the same in all directions,” Dr. Saadeh said.

“Our current understanding of the Universe is built on the assumption that it doesn’t prefer one direction over another, but there are actually a huge number of ways that Einstein’s theory of relativity would allow for space to be imbalanced. Universes that spin and stretch are entirely possible, so it’s important that we’ve shown ours is fair to all its directions.”

The researchers used measurements of the Cosmic Microwave Background (CMB) — the fossil light resulting from a time when the Universe was hot and dense, only 380,000 years after the Big Bang — taken between 2009 and 2013 by ESA’s Planck satellite.

Planck scientists recently released information about the polarization of CMB across the whole sky for the first time, providing a complementary view of the early Universe that the team was able to exploit.

Dr. Saadeh and co-authors modeled a comprehensive variety of spinning and stretching scenarios and how these might manifest in the CMB, including its polarization. They then compared their findings with the real map of the cosmos from Planck, searching for specific signs in the data.

“We calculated the different patterns that would be seen in the cosmic microwave background if space has different properties in different directions,” Dr. Saadeh said.

“Signs might include hot and cold spots from stretching along a particular axis, or even spiral distortions.”

“We then compare these predictions to reality,” said co-author Dr. Stephen Feeney, from Imperial College London.

“This is a serious challenge, as we found an enormous number of ways the Universe can be anisotropic. It’s extremely easy to become lost in this myriad of possible universes — we need to tune 32 dials to find the correct one.”

Most current cosmological studies assume that the Universe behaves identically in every direction. If this assumption were to fail, a large number of analyses of the cosmos and its content would be flawed.

“We’re very glad that our work vindicates what most cosmologists assume. For now, cosmology is safe,” Dr. Saadeh said.

The results were published this week in the journal Physical Review Letters.

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Daniela Saadeh et al. 2016. How Isotropic is the Universe? Phys. Rev. Lett. 117 (13): 131302; doi: 10.1103/PhysRevLett.117.131302