Gravitational waves

What are gravitational waves?

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What does the apparent discovery of gravitational waves by the Bicep telescope say about inflation and the big bang?
Albert Einstein

Gravitational waves are ripples in the space-time continuum, which was envisaged by Albert Einstein in his general theory of relativity. Photograph: Keystone/Getty Images

What are gravitational waves?

Gravitational waves are ripples that carry energy across the universe. They were predicted to exist by Albert Einstein in 1916 as a consequence of his General Theory of Relativity. Although there is strong circumstantial evidence for their existence, gravitational waves have not been directly detected before. This is because they are minuscule – a million times smaller than an atom. They are like tiny waves on a lake – from far away, the lake’s surface looks glassy smooth; only up very close can the details of the surface be seen.

Particularly exciting are “primordial” gravitational waves, which were generated in the first moments of the universe’s birth. These carry vital information about how the universe began.

What is general relativity?

In 1916, Albert Einstein discovered a mathematical way to explain gravity. He called it his general theory of relativity. It relied on a set of coordinates that described space and time together, known as the space-time continuum.

Matter and energy warp the space-time continuum like heavy weight on a mattress. The warping creates the force of gravity. Gravitational waves are ripples in the space-time continuum (instead of an ordinary mattress, think of a waterbed).

It isn’t all esoteric mathematics. General relativity tells us how gravity affects time, which must be taken into account by your satnav to tell you accurately where you are.

What is the significance of this discovery?

If scientists at Harvard University have detected gravitational waves, it is significant for two reasons. First, this opens up a whole new way of studying the Universe, allowing scientists to infer the processes at work that produced the waves. Second, it proves a hypothesis called inflation. This can be used to give us information about the origin of the universe, known as the big bang.

How can gravitational waves be detected?

A telescope at the south pole, called Bicep (Background Imaging of cosmic Extragalactic Polarisation), has been searching for evidence of gravitational waves by detecting a subtle property of the cosmic microwave background radiation. This radiation was produced in the big bang. It was originally discovered by American scientists in 1964 using a radio telescope and has been called the “echo” of the big bang. Bicep has measured the large-scale polarisation of this microwave radiation. Only primordial gravitational waves can imprint such a pattern, and only then if they have been amplified by inflation.

What is inflation?

The big bang was originally hypothesised by Belgian priest and physicist Georges Lemaître. He called it “the day without yesterday” because it was the moment when time and space began.

But the big bang does not fit all astronomers’ observations. The distribution of matter across space is too uniform to have come from the big bang as originally conceived. So in the 1970s, cosmologists postulated a sudden enlargement of the universe, called inflation, that occurred in the first minuscule fraction of a second after the big bang. But confirming the idea has proved difficult. Only inflation can amplify the primordial gravitational wave signal enough to make it detectable. If primordial gravitational waves have been seen, it means that inflation must have taken place.

What next? Do cosmologists just pack up and go home?

No way. Now the work really begins. Einstein knew that general relativity did not mesh with another theory of physics called quantum mechanics. Whereas general relativity talks about gravity and the universe as a whole, quantum mechanics talks about the small scale of particles and the other forces of nature, the strong and weak nuclear forces, and electromagnetism. Despite almost a century of effort, the world’s physicists have not been able to show how these theories work together. The primordial gravitational waves were generated when gravity and the universe were working on the same scale as particles and the other forces of nature. This detection and the subsequent analysis will hopefully tell us how. If it does, this could lead to what physics wistfully call “the theory of everything”.

Have physicists finally detected gravitational waves?

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Gravitational waves are the last chunk of Einstein’s General Theory of Relativity that was predicted but not yet observed. If gravitational waves have been observed, it most likely was done by the Background Imaging of Cosmic Extragalactic Polarization (Bicep) telescope at the south pole. It stared at the cosmic microwave background radiation from 2003 to 2008, but it takes a long time to process and analyze the data when looking for a faint signal in a lot of noise.

Have physicists finally detected gravitational waves?

2007 photograph of telescopes at the Dark Center at the Amundsen-Scott South Pole Station. From top to bottom, the partly-buried AST/RO, QUaD, Viper, and finally BICEP and SPT at the bottom. Image credit: Robert Schwarz

The Bicep mission page describes anticipated gravitational waves as faint, polarized, and distorted by gravitational lensing of objects between us and the cosmic microwave background radiation. They released a video of their observations in 2008. The colour scale adjusts throughout the movie to highlight temperature fluctuations of both the cosmic microwave background radiation, and the galactic plane:

Why look at the cosmic microwave background radiation for signs of gravitational waves? Because an infinitesimal moment after the universe started — 10-34 seconds after the big bang — we think it went through an inflationary period. If it did, that inflation could have amplified gravitational waves to such an extent that we can actually detect them. This would not only fill in that last missing chunk of things predicted by General Relativity that we haven’t seen yet, but also offer a glimpse into the primeval universe. They won’t be insta-proof that inflationary theory is correct, but they would rule out some cyclic theories for the origin of the universe.

Some pre-announcement articles are already mixing up very common gravity waves with gravitational waves. To differentiate, I’ll pass things off to an exasperated Dr. Katherine Mack:

People use “gravity waves” to mean “gravitational waves” constantly, so probably any clarification is a lost cause, but had to say it.

Have physicists finally detected gravitational waves?

Gravity waves are common phenomena in both the ocean and the sky, as seen in this MODIS image. Read more about them at the Earth Observatory.

As for the press conference, I’m already bracing for disappointment. “Breaking news! We’ll have breaking news for you on Monday!” announcements produce so much hype that the actual discovery probably won’t live up to expectations. I’m not the only one feeling that way — the Guardian ran an entire piece interviewing cautiously excited cosmologists warning that the observations would need to be highly robust if they’re going to be momentous.

Update: What, you can’t wait until Monday to confirm that this is all about gravitational waves before learning about them? Preposterous Universe has a detailed, lovely write-up on the topic with enough math to satisfy even pernickety cosmologists.

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Gravitational waves: have US scientists heard echoes of the big bang?

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Discovery of gravitational waves by Bicep telescope at south pole could give scientists insights into how universe was born

 

Big bang

Primordial gravitational waves would provide evidence of inflation in the moments after the big bang. Photograph: Alamy

There is intense speculation among cosmologists that a US team is on the verge of confirming they have detected “primordial gravitational waves” – an echo of the big bang in which the universe came into existence 14bn years ago.

Rumours have been rife in the physics community about an announcement due on Monday from the Harvard-Smithsonian Center for Astrophysics. If there is evidence for gravitational waves, it would be a landmark discovery that would change the face of cosmology and particle physics.

Gravitational waves are the last untested prediction of Albert Einstein’s General Theory of Relativity. They are minuscule ripples in the fabric of the universe that carry energy across space, somewhat similar to waves crossing an ocean. Convincing evidence of their discovery would almost certainly lead to a Nobel prize.

“If they do announce primordial gravitational waves on Monday, I will take a huge amount of convincing,” said Hiranya Peiris, a cosmologist from University College London. “But if they do have a robust detection … Jesus, wow! I’ll be taking next week off.”

The discovery of gravitational waves from the big bang would offer scientists their first glimpse of how the universe was born.

The signal is rumoured to have been found by a specialised telescope called Bicep (Background Imaging of Cosmic Extragalactic Polarization) at the south pole. It scans the sky at microwave frequencies, where it picks up the fossil energy from the big bang.

For decades, cosmologists have thought that the signature of primordial gravitational waves could be imprinted on this radiation. “It’s been called the Holy Grail of cosmology,” says Peiris, “It would be a real major, major, major discovery.”

Martin Hendry at the University of Glasgow works on several projects designed to directly detect gravitational waves. “If Bicep have made a detection,” he says, “it’s clear that this new window on the universe is really opening up.”

According to theory, the primordial gravitational waves will tell us about the first, infinitessimal moment of the universe’s history. Cosmologists believe that 10-34 seconds after the big bang (a decimal point followed by 33 zeros and a one) the universe was driven to expand hugely.

Known as inflation, the theory was dreamed up to explain why the universe is so remarkably uniform from place to place. But it has always lacked some credibility because no one can find a convincing physical explanation for why it happened.

Now researchers may be forced to redouble their efforts. “The primordial gravitational waves have long been thought to be the smoking gun of inflation. It’s as close to a proof of that theory as you are going to get,” says Peiris. This is because cosmologists believe only inflation can amplify the primordial gravitational waves into a detectable signal.

“If a detection has been made, it is extraordinarily exciting. This is the real big tick-box that we have been waiting for. It will tell us something incredibly fundamental about what was happening when the universe was 10-34 seconds old,” said Prof Andrew Jaffe, a cosmologist from Imperial College, London, who works on another telescope involved in the search called Polarbear.

But extracting that signal is fearsomely tricky. The microwaves that carry it must cross the whole universe before arriving at Earth. During the journey, they are distorted by intervening clusters of galaxies.

“It’s like looking at the universe through bubbled glass,” said Duncan Hanson of McGill University in Montreal, Canada, who works on the South Pole Telescope, a rival that sits next to Bicep.

He said the distortion must be removed in a convincing way before anyone can claim to have made the detection. The prize for doing that, however, would be the pinnacle of a scientific career. “The Nobel Prize would be for the detection of the primordial gravitational waves.”

“Yeah, I would give them a prize,” said Jaffe.