Flooring the cosmic accelerator

New measure of dark energy suggests ‘Big Rip’ for universe

BALTIMORE — Let ’er rip!

TORN ASUNDER In some 60 billion years, according to the Big Rip model, the entire universe, down to the smallest atom, will be torn apart by a proposed form of dark energy that grows stronger with time. NASA, ESA, Hubble Heritage, STScI/AURA, T. Ghose

If cosmologist Will Percival of the University of Portsmouth in England is right, the universe will end about 60 billion years from now, when every molecule and atom will be torn asunder by a mysterious entity that opposes gravity’s pull and turns it into a cosmic push.

The cosmic killer is a runaway version of what astronomers call dark energy, an unidentified substance that pervades all of space. Dark energy appears to cause the universe to expand at an accelerated rate. Many studies have found hints that the density of dark energy is constant over time and that it therefore exerts a constant repulsive force.

But Percival and his collaborators, studying cosmic expansion by measuring sound waves generated in the early universe, have found the first sign that dark energy could be growing stronger over time. It’s as if someone had floored the cosmic gas pedal. And that would lead to a universe that ends in the Big Rip.

Percival’s team examined the echo of sound waves created soon after the Big Bang, when photons and baryons — ordinary subatomic particles like electrons — were bound together. The primordial cosmic sound-wave oscillations arose because of the tug of war between gravity, which acted to compress each photon-baryon clump, and the radiation pressure exerted by the photons, which resisted that clumping. Although the sound waves ceased some 400,000 years after the Big Bang, when the universe became cool enough for photons and baryons to go their separate ways, the echoes of this cosmic symphony left their imprint as ripples in the distribution of galaxies in the modern-day universe. The characteristic wavelength of these ripples provides a standard ruler for gauging cosmic expansion.

Using data from two large galaxy surveys, Percival and his colleagues identified the acoustic oscillations at two different epochs, 3.83 billion years ago and 2.4 billion years ago. The oscillations were much more spread out at the recent epoch — so much so that they could only be accounted for if dark energy had grown stronger, stretching space much faster than it had at the earlier time, Percival says.

He reported the preliminary finding May 7 in Baltimore at a meeting on dark energy at the Space Telescope Science Institute.

Dark energy that grows stronger with time, dubbed phantom energy by theorist Robert Caldwell of DartmouthCollege, isn’t popular with physicists because no known theory can explain it.

“It’s almost heresy” to find evidence for phantom energy, Percival says.

Phantom energy would cause a runaway expansion, in which the universe would become infinitely large and time would effectively cease at a cosmic age of 70 billion years, according to calculations by Caldwell and Marc Kamionkowski of the California Institute of Technology in Pasadena and Nevin N. Weinberg at the University of California, Berkeley.

A billion years before the absolute end, phantom energy would tear apart clusters of galaxies. The Milky Way would succumb about 100 million years before the Big Rip. A few months before the end of time, the dark energy content of the empty space between Earth and the sun would overwhelm the sun’s pull, and Earth would float off into space. An hour before the end, Earth itself would fall apart. Finally, 10-19 seconds before the Big Rip, molecules and atoms would break up.

Percival’s study appears to be in conflict with another new study measuring the current expansion rate of the universe. Known as the Hubble constant, the expansion rate can be used to estimate the age of the universe and the character of dark energy.

A new measuring method has reduced uncertainty in the constant’s value by more than half, to 4.8 percent, Adam Riess of the Space Telescope Science Institute reported May 5. The new value, 74 kilometers per second per megaparsec (plus or minus 3.5), indicates that the universe is about 350 million years younger than the previous 13.7 billion–year estimate. It also hints that dark energy has a constant density, a result “in tension” with Percival’s results, Riess notes.

To obtain the Hubble constant, astronomers need to measure the distances to galaxies. The standard way to do so has been to find a source whose intrinsic brightness can be easily calculated. By comparing the object’s intrinsic brightness with how dim it appears on the sky, astronomers can determine how far away its home galaxy must lie. But this method isn’t as precise as astronomers would like.

That prompted Riess and other astronomers to find a new way to measure galactic distances. Their method relies on tracking intense radio emissions from water molecules that lie within the swirling disk of gas surrounding a supermassive black hole at the heart of a relatively nearby galaxy, NGC 4258.

“What’s really new is that the Hubble constant with this precision level has become competitive” with other methods, including primordial cosmic sound-wave oscillations, for measuring dark energy, Riess says.

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