Are there gravitational waves

If detectors could measure these ripples in space-time, emanating from interacting pairs of distant objects, scientists would have all the information needed to calculate how strong the signal was to start with — and so how far the waves must have travelled to reach Earth.

Thus, he predicted, gravitational waves could be unambiguous markers of how quickly the Universe is expanding. His idea was elegant but impractical: nobody at the time could detect gravitational waves.

But, last August, Schutz finally got the opportunity to test this concept when the reverberations of a million-year-old merger between two neutron stars passed through gravitational-wave detectors on Earth. As luck would have it, the event occurred in a relatively nearby galaxy, producing a much cleaner first measure than Schutz had dreamed. With that one data point, Schutz was able to show that his technique could become one of the most reliable for measuring distance. How gravitational waves might help fundamental cosmology.

More mergers like that one could help researchers to resolve an ongoing debate over how fast the Universe currently is expanding. But cosmology is just one discipline that could make big gains through detections of gravitational waves in the coming years. Gravitational waves might even provide a window into what happened in the first few moments after the Big Bang. Like many scientists, Schutz hopes that the best discoveries will be ones that no theorist has even dreamed of.

For a field of research that is not yet three years old, gravitational-wave astronomy has delivered discoveries at a staggering rate, outpacing even the rosiest expectations. The discoveries are the most direct proof yet that black holes truly exist and have the properties predicted by general relativity. They have also revealed, for the first time, pairs of black holes orbiting each other. Researchers now hope to find out how such pairings came to be. The individual black holes in each pair should form when massive stars run out of fuel in their cores and collapse, unleashing a supernova explosion and leaving behind a black hole with a mass ranging from a few to a few dozen Suns.

Or, the black holes might form independently, but be driven together later by frequent gravitational interactions with other objects — something that could happen in the centres of dense star clusters.

Ilya Mandel, a theoretical astrophysicist at the University of Birmingham, UK, says that for LIGO and Virgo to see such pairs merge, typical black holes need to have started their mutual orbit separated by a distance of less than one-quarter that between Earth and the Sun. The five black-hole mergers discovered so far are not sufficient to determine which formation scenario dominates. But in an August analysis of the first three detections, a group including Mandel and Will Farr, a theoretical astrophysicist and LIGO member at the University of Birmingham, suggested that just ten more observations could provide substantial evidence in favour of one scenario or the other 1.

This would involve scrutinizing the gravitational waves for clues about how black holes rotate: those that pair up after forming independently should have randomly oriented spins, whereas those with a common origin should have spin axes that are parallel to each other and roughly perpendicular to the plane in which they orbit.

Further observations could also provide insight into some of the fundamental questions about black-hole formation and stellar evolution. Collecting many measurements of masses should reveal gaps — ranges in which few or no black holes exist, says Vicky Kalogera, a LIGO astrophysicist at Northwestern University in Evanston, Illinois.

And at the high end — around 50 times the mass of the Sun — researchers expect to see another cut-off. In very large stars, pressures at the core are thought eventually to produce antimatter, causing an explosion so violent that the star simply disintegrates without leaving any remnants at all.

These events, called pair-instability supernovae, have been theorized, but so far there has been scant observational evidence to back them up. How to hunt for a black hole with a telescope the size of Earth. Eventually, the black-hole detections will delineate a map of the Universe in the way galaxy surveys currently do, says Rainer Weiss, a physicist at the Massachusetts Institute of Technology in Cambridge who was the principal designer of LIGO.

To ramp up these observations, LIGO and Virgo have plans to improve their sensitivity, which will reveal not only more events, but also more details about each merger.

Having more observatories spread around the globe will also be crucial. KAGRA, a detector under construction deep underground in Japan, might start gathering data by late And India is planning to build another observatory in the next decade, made in part with spare components from LIGO.

An even bigger trove of discoveries could come from observing neutron-star mergers. So far, researchers have announced only one such detection, called GW Further observations could allow scientists to explore the interiors of these objects. For a more detailed discussion of this discovery and work, see Look Deeper. Artist's Impression of a Binary Pulsar. Since then, many astronomers have studied pulsar radio-emissions pulsars are neutron stars that emit beams of radio waves and found similar effects, further confirming the existence of gravitational waves.

But these confirmations had always come indirectly or mathematically and not through direct contact. All of this changed on September 14, , when LIGO physically sensed the undulations in spacetime caused by gravitational waves generated by two colliding black holes 1. LIGO's discovery will go down in history as one of humanity's greatest scientific achievements.

While the processes that generate gravitational waves can be extremely violent and destructive, by the time the waves reach Earth they are thousands of billions of times smaller!

In fact, by the time gravitational waves from LIGO's first detection reached us, the amount of space-time wobbling they generated was a times smaller than the nucleus of an atom! Such inconceivably small measurements are what LIGO was designed to make. Over the history of the Universe, galaxies have collided together, making even bigger galaxies. In these collisions, the black holes from the galaxy centers paired up, sending out gravitational waves that have a period the time between each wave peak of years to decades.

Since black holes do not emit any light, the only way to detect them is with gravitational waves. Measuring gravitational waves is a radically new way of observing the Universe and these measurements will tell us more about the true nature of gravity. A pulsar is a special kind of neutron star that spins around very quickly as quickly as hundreds of times each second and shoots out beams of radio waves see the left side of Figure 4. Pulsars are very reliable; we can very accurately predict when the radio pulses will arrive at Earth.

This means we can use pulsars like a stopwatch, with which we mark the passage of time by the number of radio pulses that have been observed from a pulsar. Pulsars are great stopwatches that stay reliable over many years. If a gravitational wave crosses the space between Earth and a pulsar, it will stretch and squeeze that space.

If space is stretched, it will take longer than expected for the radio beam to reach us; the pulse will arrive late! The opposite is true if space is squeezed, because the radio pulse will arrive earlier than expected.

We can subtract our predictions of when the radio pulses should arrive from our real observations, and look at the difference.

That difference could be due to gravitational waves! In NANOGrav's most recent hunt [ 5 ], we made a net out of 34 of these pulsars that have been watched by astronomers every couple of weeks over the last 11 years. We did not find any gravitational waves, but we also know that our signals take a long time to stick out above all of the noise and weird stuff that can affect pulsars.

Even though we have not detected anything yet, we think it will only be another 3 years, or possibly 7 years at the most, before we do [ 6 ].

We may not have seen any waves yet, but this absence of waves has allowed us to disprove predictions made by other scientists who thought we should have seen something by now. Our data will help those scientists to revise and update their predictions.

The results we have obtained also help us understand how often massive black holes merge together in the Universe. We also found that gravitational waves with periods of 1 year cause stretches and squeezes to space that are very, very tiny—so small that the change they cause to the size of the Earth is only about 10 times the width of a human DNA strand see Figure 5!

We expect the future of gravitational-wave astronomy to be very exciting, allowing us to peer into parts of the Universe that other telescopes cannot see. The detection of gravitational waves by pulsar-timing arrays in the near future will be a huge discovery.

In NANOGrav, we are checking for other types of gravitational waves in this net of pulsars and will report on them over the next year. We are also constantly searching for new pulsars to fill holes in our net, so that we are better able to find gravitational waves. Our results so far have been fascinating, and we are preparing for the day quite soon when we can tell the world that we have seen gravitational waves from the most massive black holes in the entire Universe!

This is a pair of large gravitational-wave detectors located in Hanford, Washington and Livingston, Louisiana. They detected gravitational waves for the first time in