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Did the early cosmos have one dimension?

Artist's impression of gravitational waves

A controversial theory suggests there was only one dimension right after the Big Bang. Extra dimensions formed as time progressed, leading to our present three-dimensional universe.

DID THE EARLY UNIVERSE have just one spatial dimension? That’s the mind-boggling concept at the heart of a theory that University at Buffalo physicist Dejan Stojkovic and colleagues proposed in 2010.

They suggested that the early universe—which exploded from a single point and was very, very small at first—was one-dimensional (like a straight line) before expanding to include two dimensions (like a plane) and then three (like the world in which we live today).

The concept, if valid, would address important problems in particle physics.

Now, in a new paper in Physical Review Letters, Stojkovic and Loyola Marymount University physicist Jonas Mureika describe a test that could prove or disprove the “vanishing dimensions” hypothesis.

Because it takes time for light and other waves to travel to Earth, telescopes peering out into space can, essentially, look back in time as they probe the universe’s outer reaches.

Gravitational waves can’t exist in one- or two-dimensional space. So Stojkovic and Mureika have reasoned that the Laser Interferometer Space Antenna (LISA), a planned international gravitational observatory, should not detect any gravitational waves emanating from the lower-dimensional, early years of the early universe.

Artist's impression of LISA

Artist's impression of the planned Laser Interferometer Space Antenna, which could will search for gravitational waves.

Quantum mechanics vs general relativity

Stojkovic, an assistant professor of physics, says the theory of evolving dimensions represents a radical shift from the way we think about the cosmos—about how our universe came to be.

The core idea is that the dimensionality of space depends on the size of the space we’re observing, with smaller spaces associated with fewer dimensions. That means that a fourth dimension will open up—if it hasn’t already—as the universe continues to expand.

The theory also suggests that space has fewer dimensions at very high energies of the kind that would have been around in the early, post-big bang universe.

If Stojkovic and his colleagues are right, they’ll be helping to address fundamental problems with the standard model of particle physics.

One of those problems is the incompatibility between quantum mechanics and general relativity.

Quantum mechanics and general relativity are mathematical frameworks that describe the physics of the universe. Quantum mechanics is good at describing the universe at very small scales (such as atoms), while relativity is good at describing the universe at the largest scales.

Albert Einstein

Einstein's general relativity theory and quantum mechanics are at odds with each other.

Currently, the two theories are considered incompatible. But if the universe, at its smallest levels, had fewer dimensions, mathematical discrepancies between the two frameworks would disappear.

Something radically wrong?

The second problem is the mystery of the universe’s accelerating expansion.

Physicists have observed that the expansion of the universe seems to be speeding up, and they don’t know why. The addition of new dimensions as the universe grows would explain this acceleration. (Stojkovic says a fourth dimension may have already opened at large, cosmological scales.)

Another problem is the need to alter the mass of the Higgs boson…an as yet undiscovered elementary particle predicted by the standard model of particle physics.

For equations in the standard model to accurately describe the observed physics of the real world, however, researchers must artificially adjust the mass of the Higgs boson for interactions between particles that take place at high energies. If space has fewer dimensions at high energies, the need for this kind of “tuning” disappears.

“What we’re proposing here is a shift in paradigm,” Stojkovic said. “Physicists have struggled with the same problems for 10, 20, 30 years, and straight-forward extensions of extensions of the existing ideas are unlikely to solve them.”

“We have to take into account the possibility that something is systematically wrong with our ideas,” he continued. “We need something radical and new, and this is something radical and new.”

Artist's impression of the Big Bang

Artist's impression of the Big Bang

Lower-dimensional space

Because the planned deployment of LISA is still years away, it may be a long time before Stojkovic and his colleagues are able to test their ideas this way.

However, some experimental evidence already points to the possible existence of lower-dimensional space.

Specifically, scientists have observed that the main energy flux of cosmic ray particles with the kind of high energy associated with the very early universe, are aligned along a two-dimensional plane.

If high energies do correspond with lower-dimensional space, as the ‘vanishing dimensions’ theory proposes, researchers working with the Large Hadron Collider particle accelerator in Europe should see ‘planar scattering’ at such energies.

Stojkovic says the observation of such events would be “a very exciting, independent test of our proposed ideas.”

Adapted from information issued by the University at Buffalo.

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Hungry black holes shred stars

Artist's conception of black holes about to merge

In this artist's conception, two black holes are about to merge. When they combine, gravitational wave radiation will "kick" the new, bigger black hole like a rocket engine, sending it rampaging through nearby stars.

A GALAXY’S CORE IS A BUSY PLACE, crowded with stars swarming around an enormous black hole. When galaxies collide (as they sometimes do), it gets even messier as the galaxies’ black holes spiral toward each other, merging to form an even bigger gravitational monster.

Once it is formed, the monster goes on a rampage, zooming into the surrounding starfields. There it finds a hearty meal, shredding and swallowing stars at a rapid rate.

According to new research by Nick Stone and Avi Loeb (Harvard-Smithsonian Centre for Astrophysics), upcoming sky surveys might offer astronomers a way to catch one of these gorging black holes “in the act.”

Before the merger, as the two black holes whirl around each other, they stir the galactic centre like the blades of a blender. Their strong gravity warps space, sending out ripples known as gravitational waves.

When the black holes merge, they emit gravitational waves more strongly in one direction. That inequality kicks the new black hole into motion in the opposite direction like a rocket engine.

“That kick is very important. It can shove the black hole toward stars that otherwise would have been at a safe distance,” said Stone.

“Essentially, the black hole can go from starving to enjoying an all-you-can-eat buffet,” he added.

Spotting a dying star

When tidal forces rip a star apart, its remains spiral around the black hole, smashing and rubbing together, heating up enough to shine in the ultraviolet or X-rays. This region immediately surrounding the black hole will glow as brightly as an exploding star, or supernova, before gradually fading in a distinctive way.

Artist's conception of the Laser Interferometer Space Antenna

Artist's conception of the Laser Interferometer Space Antenna, a trio of spacecraft that should be able to pick up gravitational waves from merging black holes.

Importantly, a wandering, supermassive black hole is expected to swallow many more stars than a black hole in an undisrupted galactic centre.

A stationary black hole disrupts one star every 100,000 years. But in the best-case scenario, a wandering black hole could disrupt a star every decade.

Astronomers would have a much better change of spotting the latter events, particularly with new survey facilities like Pan-STARRS and the Large Synoptic Survey Telescope.

The siren call of gravity

Catching the dying scream from a disrupted star is a good start. However, astronomers really want to combine that information with gravitational wave data from the black hole merger.

The Laser Interferometer Space Antenna (LISA), a future mission designed to detect and study gravitational waves, could provide that data.

Gravitational wave measurements can potentially yield very accurate distance measurements (to better than one part in a hundred, or 1 percent) to the scene of the black hole crime. However, they won’t be able to provide precise sky direction co-ordinates.

Spotting a star’s tidal disruption will let astronomers pinpoint the galaxy containing the recently merged black-hole binary, thus providing the direction.

By correlating the host galaxy’s redshift (a change in its light caused by the expanding universe) with an accurate distance, astronomers can infer the ‘equation of state’ of dark energy.

In other words, they can learn more about the force that’s accelerating cosmic expansion, and which dominates the cosmic mass/energy budget today.

“Instead of ‘standard candles’ like supernovae, the black hole binary would be a ‘standard siren.’ Using it, we could create the most accurate cosmic ‘ruler’ possible,” stated Loeb.

Adapted from information issued by CfA. Black hole artwork by David A. Aguilar (CfA).

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