Beyond Spacetime: Gravitational Waves Might Reveal Extra-Dimensions

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NASA (edit by Ian O’Neill)

We are well and truly on our way to a new kind of astronomy that will use gravitational waves — and not electromagnetic waves (i.e. light) — to “see” a side of the universe that would otherwise be invisible.

From crashing black holes to wobbling neutron stars, these cosmic phenomena generate ripples in spacetime and not necessarily emissions in the electromagnetic spectrum. So when the Laser Interferometer Gravitational-wave Observatory (LIGO) made its first gravitational wave detection in September 2015, the science world became very excited about the reality of “gravitational wave astronomy” and the prospect of detecting some of the most massive collisions that happen in the dark, billions of light-years away.

Like waves rippling over the surface of the ocean, gravitational waves travel through spacetime, a prediction that was made by Albert Einstein over a century ago. And like those ocean waves, gravitational waves might reveal something about the nature of spacetime.

We’re talking extra-dimensions and a new study suggests that gravitational waves may carry an awful lot more information with them beyond the characteristics of what generated them in the first place.

Our 4-D Playing Field

First things first, remember that we interact only with four-dimensional spacetime: three dimensions of space and one dimension of time. This is our playing field; we couldn’t care less whether there are more dimensions out there.

Unless you’re a physicist, that is.

And physicists are having a hard job describing gravity, to put it mildly. This might seem weird considering how essential gravity is for, well, everything. Without gravity, no stars would form, planets wouldn’t coalesce and the cosmos would be an exceedingly boring place. But gravity doesn’t seem to “fit” with the Standard Model of physics. The “recipe” for the universe is perfect, except it’s missing one vital ingredient: Gravity. (It’s as if a perfect cake recipe is missing one crucial ingredient, like flour.)

There’s another weird thing about gravity: Although it’s very important in our universe (yes, there might be more than one universe, but I’ll get to that later), it is actually the weakest of all forces.

But why so weak? This is where string theory comes in.

String theory (and, by extension, superstring theory) predicts that the universe is composed of strings that vibrate at different frequencies. These strings form something like a vast, superfine noodle soup and these strings thread through many dimensions (many more than our four-dimensions) creating all the particles and forces that we know and love.

Now, the possible reason why gravity is so weak when compared with the other fundamental forces could be that gravity is interacting with many more dimensions that are invisible to us 4-D beings. Although string theory is a wonderful mathematical tool to describe this possibility, there is little physical evidence to back up this superfine noodly mess, however.

But as already mentioned, if string theory holds true, it would mean that our universe contains many more dimensions than we regularly experience. (The unifying superstring theory, called “M-theory”, predicts a total of 11 dimensions and may provide the framework that unifies the fundamental forces and could be the diving board that launches us into the vast ocean that is the multiversebut I’ll stop there, I’ve said too much.)

Groovy. But what the heck has this got to do with gravitational waves? As gravitational waves travel through spacetime, they might be imprinted with information about these extra dimensions. Like our wave analogy, as the sea washes over a beach, the frequency of the waves increase as the water becomes shallower — the ocean waves are imprinted with information about how deep the water is. Could gravitational waves washing over (or, more accurately, through) spacetime also create some kind of signature that would reveal the presence of very, very tiny extra-dimensions as predicted by superstring theory?

Possibly, say researchers at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) in Potsdam, Germany.

“Physicists have been looking for extra dimensions at the Large Hadron Collider at CERN but up to now this search has yielded no results,” says Gustavo Lucena Gómez, second author of a new study published in the Journal of Cosmology and Astroparticle Physics. “But gravitational wave detectors might be able to provide experimental evidence.”

Beyond Spacetime?

The researchers suggest that these extra-dimensions might modify the signal of gravitational waves received by detectors like LIGO and leave a very high-frequency “fingerprint.” But as this frequency would be exceedingly high — of the order of 1000 Hz — it’s not conceivable that the current (and near-future) ground-based gravitational wave detectors will be sensitive enough to even hope to detect these frequencies.

However, extra-dimensions might modify the gravitational waves in a different way. As gravitational waves propagate, they stretch and shrink the spacetime they travel through, like this:

gw-waves-wave

The amount of spacetime warping might therefore be detected as more gravitational wave detectors are added to the global network. Currently, LIGO has two operating observing stations (one in Washington and one in Louisiana) and next year, the European Virgo detector will start taking data.

More detectors are planned elsewhere, so it’s possible that we may, one day, use gravitational waves to not only “see” black holes go bump in the night, we might also “see” the extra-dimensions that form the minuscule tapestry of the fabric beyond spacetime. And if we can do this, perhaps we’ll finally understand why gravity is so weak and how it really fits in with the Standard Model of physics.

Want to know more about gravitational waves? Well, here’s an Astroengine YouTube video on the topic:

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When Black Holes Collide… Astroengine Is Now On YouTube!

So… it begins!

Astroengine has finally been launched on YouTube, kicking off with a summary of the recent gravitational wave discovery by LIGO. I’m aiming to produce at least one video a week and I’d really like to make it as viewer-driven as possible. So if you have any burning space science questions or any critique about the videos I’m posting, please reach out!

But for now, you know what to do: like, subscribe and enjoy!

This Super-Hot, Super-Weird Space Doughnut Could Be a New “Planetary Object”

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The structure of a planet, a planet with a disk and a synestia, all of the same mass (Simon Lock and Sarah Stewart)

Pluto is going to be pissed.

After studying computer simulations of planetary collisions, scientists have discovered a possible phase of planetary formation that has, so far, been overlooked by astronomy. And they think this phase is so significant that it deserves its own planetary definition.

After two planetary objects collide, researchers from the University of California Davis and Harvard University in Cambridge, Mass., realized that a bloated, spinning mass of molten rock can form. It looks a bit like a ring doughnut with the hole filled in. What’s more, it is thought that Earth (and other planets in the solar system) probably went through this violent period before becoming the solid spinning globes we know and love today.

They call this partly vaporized rock “synestia” — “syn-” for “together” and “Estia” after the Greek goddess of architecture and structures.

Over a range of masses and collision speeds, planetary scientist Sarah Stewart (Davis) and graduate student Simon Lock (Harvard) simulated planetary collisions and focused on how the angular momentum of colliding bodies might influence the system. Their study has been published in the Journal of Geophysical Research: Planets. Basically, when two bodies — with their own angular momentum — collide and merge, the sum of their momenta must be conserved and this can have a dramatic effect on the size and structure of the merged mass.

“We looked at the statistics of giant impacts, and we found that they can form a completely new structure,” said Stewart.

After colliding, the energetic event causes both planets to melt and partially vaporize, expanding with a connected ring-like structure. And this structure — a synestia — would eventually cool, contract and solidify. It could also form moons; post-collision molten debris in the synestia doughnut ring may emerge in a stable orbit around the planet.

The synestia phase would be a fleeting event in any planet’s life, however. For an Earth-sized mass, the post-collision synestia would likely last only 100 years or so. But the larger the mass, the longer the phase, the researchers theorize.

So, giving this theoretical “planetary object” a classification might be a little generous — a move that would raise recently “demoted” Pluto’s eyebrow — but as telescopes become more advanced, we might one day be lucky enough to spy a synestia in a young star system where dynamic instabilities are causing planets to careen into one another.

We’re Really Confused Why Supermassive Black Holes Exist at the Dawn of the Cosmos

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ESO

Supermassive black holes can be millions to billions of times the mass of our sun. To grow this big, you’d think these gravitational behemoths would need a lot of time to grow. But you’d be wrong.

When looking back into the dawn of our universe, astronomers can see these monsters pumping out huge quantities of radiation as they consume stellar material. Known as quasars, these objects are the centers of primordial galaxies with supermassive black holes at their hearts.

Now, using the twin W. M. Keck Observatory telescopes on Hawaii, researchers have found three quasars all with billion solar mass supermassive black holes in their cores. This is a puzzle; all three quasars have apparently been active for short periods and exist in an epoch when the universe was less than a billion years old.

Currently, astrophysical models of black hole accretion (basically models of how fast black holes consume matter — likes gas, dust, stars and anything else that might stray too close) woefully overestimate how long it takes for black holes to grow to supermassive proportions. What’s more, by studying the region surrounding these quasars, researchers at the Max Planck Institute for Astronomy (MPIA) in Germany have found that these quasars have been active for less than 100,000 years.

To put it mildly, this makes no sense.

“We don’t understand how these young quasars could have grown the supermassive black holes that power them in such a short time,” said lead author Christina Eilers, a post-doctorate student at MPIA.

Using Keck, the team could take some surprisingly precise measurements of the quasar light, thereby revealing the conditions of the environment surrounding these bright baby galaxies.

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MPIA

Models predict that after forming, quasars began funneling huge quantities of matter into the central black holes. In the early universe, there was a lot of matter in these baby galaxies, so the matter was rapidly consumed. This created superheated accretion disks that throbbed with powerful radiation. The radiation blew away a comparatively empty region surrounding the quasar called a “proximity zone.” The larger the proximity zone, the longer the quasar had been active and therefore the size of this zone can be used to gauge the age (and therefore mass) of the black hole.

But the proximity zones measured around these quasars revealed activity spanning less than 100,000 years. This is a heartbeat in cosmic time and nowhere near enough time for a black hole pack on the supermassive pounds.

“No current theoretical models can explain the existence of these objects,” said Joseph Hennawi, who led the MPIA team. “The discovery of these young objects challenges the existing theories of black hole formation and will require new models to better understand how black holes and galaxies formed.”

The researchers now hope to track down more of these ancient quasars and measure their proximity zones in case these three objects are a fluke. But this latest twist in the nature of supermassive black holes has only added to the mystery of how they grow to be so big and how they relate to their host galaxies.

Supermassive black hole with torn-apart star (artist’s impress
A supermassive black hole consumes a star in this artist’s impression (ESO)

These questions will undoubtedly reach fever-pitch later this year when the Event Horizon Telescope (EHT) releases the first radio images of the 4 million solar mass black hole lurking at the center of our galaxy. Although it’s a relative light-weight among supermassives, direct observations of Sagittarius A* may uncover some surprises as well as confirm astrophysical models.

But as for how supermassive black holes can possibly exist at the dawn of our universe, we’re obviously missing something — a fact that is as exciting as it is confounding.

Plasma ‘Soup’ May Have Allowed Ancient Black Holes to Beef up to Supermassive Proportions

How ancient supermassive black holes grew so big so quickly is one of the biggest mysteries hanging over astronomy — but now researchers think they know how these behemoths packed on the pounds.

John Wise, Georgia Tech

Supermassive black holes are the most extreme objects in the universe. They can grow to billions of solar masses and live in the centers of the majority of galaxies. Their extreme gravities are legendary and have the overwhelming power to switch galactic star formation on and off.

But as our techniques have become more advanced, allowing us to look farther back in time and deeper into the distant universe, astronomers have found these black hole behemoths lurking, some of which are hundreds of millions to billions of solar masses. This doesn’t make much sense; if these objects slowly grow by swallowing cosmic dust, gas, stars and planets, how did they have time only a few hundred million years after the Big Bang to pack on all those pounds?

Well, when the universe was young, it was a very different place. Closely-packed baby galaxies generated huge quantities of radiation and this radiation had a powerful influence over star formation processes in neighboring galaxies. It is thought that massive starburst galaxies (i.e. a galaxy that is dominated by stellar birth) could produce so much radiation that they would, literally, snuff-out star formation in neighboring galaxies.

Stars form in vast clouds of cooling molecular hydrogen and, when star birth reigns supreme in a galaxy, black holes have a hard time accreting matter to bulk up — these newly-formed stars are able to escape the black hole’s gravitational grasp. But in the ancient universe, should a galaxy that is filled with molecular hydrogen be situated too close to a massive, highly radiating galaxy, these clouds of molecular hydrogen could be broken down, creating clouds of ionized hydrogen plasma — stuff that isn’t so great for star formation. And this material can be rapidly consumed by a black hole.

According to computer simulations of these primordial galaxies of hydrogen plasma, if any black hole is present in the center of that galaxy, it will feed off this plasma “soup” at an astonishingly fast rate. These simulations are described in a study published in the journal Nature Astronomy.

“The collapse of the galaxy and the formation of a million-solar-mass black hole takes 100,000 years — a blip in cosmic time,” said astronomer Zoltan Haiman, of Columbia University, New York. “A few hundred-million years later, it has grown into a billion-solar-mass supermassive black hole. This is much faster than we expected.”

But for these molecular hydrogen clouds to be broken down, the neighboring galaxy needs to be at just the right distance to “cook” its galactic neighbor, according to simulations that were run for several days on a supercomputer.

“The nearby galaxy can’t be too close, or too far away, and like the Goldilocks principle, too hot or too cold,” said astrophysicist John Wise, of the Georgia Institute of Technology.

The researchers now hope to use NASA’s James Webb Space Telescope, which is scheduled for launch next year, to look back to this era of rapid black hole formation, with hopes of actually seeing these black hole feeding processes in action. Should observations agree with these simulations, we may finally have some understanding of how these black hole behemoths grew so big so quickly.

“Understanding how supermassive black holes form tells us how galaxies, including our own, form and evolve, and ultimately, tells us more about the universe in which we live,” added postdoctoral researcher John Regan, of Dublin City University, Ireland.

This Black Hole Keeps Its Own White Dwarf ‘Pet’

The most compact star-black hole binary has been discovered, but the star seems to be perfectly happy whirling around the massive singularity twice an hour.

Credits: X-ray: NASA/CXC/University of Alberta/A.Bahramian et al.; Illustration: NASA/CXC/M.Weiss

A star in the globular cluster of 47 Tucanae is living on the edge of oblivion.

Located near a stellar-mass black hole at only 2.5 times the Earth-moon distance, the white dwarf appears to be in a stable orbit, but it’s still paying the price for being so intimate with its gravitational master. As observed by NASA’s Chandra X-ray Observatory and NuSTAR space telescope, plus the Australia Telescope Compact Array, gas is being pulled from the white dwarf, which then spirals into the black hole’s super-heated accretion disk.

47 Tucanae is located in our galaxy, around 14,800 light-years from Earth.

Eventually, the white dwarf will become so depleted of plasma that it will turn into some kind of exotic planetary-mass body or it will simply evaporate away. But one thing does appear certain, the white dwarf will remain in orbit and isn’t likely to get swallowed by the black hole whole any time soon.

“This white dwarf is so close to the black hole that material is being pulled away from the star and dumped onto a disk of matter around the black hole before falling in,” said Arash Bahramian, of the University of Alberta (Canada) and Michigan State University. “Luckily for this star, we don’t think it will follow this path into oblivion, but instead will stay in orbit.” Bahramian is the lead author of the study to be published in the journal Monthly Notices of the Royal Astronomical Society.

It was long thought that globular clusters were bad locations to find black holes, but the 2015 discovery of the binary system — called “X9” — generating quantities of radio waves inside 47 Tucanae piqued astronomers’ interest. Follow-up studies revealed fluctuating X-ray emissions with a period of around 28 minutes — the approximate orbital period of the white dwarf around the black hole.

So, how did the white dwarf become the pet of this black hole?

The leading theory is that the black hole collided with an old red giant star. In this scenario, the black hole would have quickly ripped away the bloated star’s outer layers, leaving a tiny stellar remnant — a white dwarf — in its wake. The white dwarf then became the black hole’s gravitational captive, forever trapped in its gravitational grasp. Its orbit would have become more and more compact as the system generated gravitational waves (i.e. ripples in space-time), radiating orbital energy away, shrinking its orbital distance to the configuration that it is in today.

It is now hoped that more binary systems of this kind will be found, perhaps revealing that globular clusters are in fact very good places to find black holes enslaving other stars.