Pulsars Help the Search for Gravitational Waves - Video

In a new video from Physics World, scientists from the Jodrell Bank Observatory near Manchester in the United Kingdom discuss how natural time keepers called pulsars, which are actually the condensed left-over material from burned-out stars, could help the search for gravitational waves.

Einstein predicted that very massive, energetic events (like two stars merging together) could create ripples in the fabric of space, the way a stone creates ripples on the surface of a pond.

The ripples aren't made of light or matter, but of space itself. These are called gravitational waves.

So far, astronomers have not been able to detect gravitational waves directly, and some astronomers are choosing to take an indirect path.

Pulsars earned their name because their light appears to pulse on and off. In some cases, the pulsar's blinking is so regular, it exceeds the precision of any clock that can be built by humans.

An interruption in that regularity, therefore, must come from an external event. Check out the video to see how interruptions to these regular pulses of light could indicate the presence of gravitational waves.

SPIDER: 'Spacecraft' seeks traces of the early universe over Antartica

Constructed primarily in Princeton's Jadwin Hall, SPIDER is a stratospheric spacecraft that in December will begin a 20-day orbit in Earth's stratosphere at an altitude of roughly 110,000 feet.

During that period, SPIDER's six large cameras will look for the pattern, or polarization, of gravitational waves produced by the fluctuation of energy and density that resulted from the Big Bang.

These waves, explained William Jones a Princeton University assistant professor of physics, are a "statistically unique fingerprint" that can be traced back to the beginning of the universe.

Many astronomical instruments measure various characteristics of this fingerprint, SPIDER is designed to characterize the "shape" of it, said Jones, who is the project's principal investigator.

"The ultimate goal of SPIDER is to see to what extent we can identify a very characteristic feature in that polarization that's expected to come from the earliest stages of the evolutionary growth of our universe," Jones said.

"There's a very particular pattern than can be generated only by something like a gravitational wave propagating through the surface of the cosmic microwave background [which is the glow of the heat left over from the Big Bang]," Jones said.

"That is a very particular pattern commonly referred to as a 'pinwheel' pattern on the sky. It's that particular pinwheel pattern that we're really after."

SPIDER, which used to be an acronym, but now is the project's formal name, is a multi-institutional project funded largely by a grant from NASA, as well as the David and Lucille Packard Foundation.

In addition to Princeton, the primary institutions involved are the University of Toronto; Case Western Reserve University; the California Institute of Technology and the Jet Propulsion Laboratory, a NASA-funded research center managed by Caltech; and the University of British Columbia.

The project was proposed in 2006 while Jones, who joined Princeton's faculty in 2008, was a scientist at the Jet Propulsion Laboratory.

Hints of gravitational waves found in the stars

Energetic events, such as this artist’s rendition of a binary-star merger, are thought to create gravitational waves that cause ripples in space and time. 

 Credit: NASA

Scientists have shown how gravitational waves, invisible ripples in the fabric of space and time that propagate through the universe, might be "seen" by looking at the stars.

The new model proposes that a star that oscillates at the same frequency as a gravitational wave will absorb energy from that wave and brighten, an overlooked prediction of Einstein's 1916 theory of general relativity.

The study, which was published today in the Monthly Notices of the Royal Astronomical Society: Letters, contradicts previous assumptions about the behavior of gravitational waves.

"It's pretty cool that a hundred years after Einstein proposed this theory, we're still finding hidden gems," said Barry McKernan, a research associate in the Museum's Department of Astrophysics, who is also a professor at CUNY's Borough of Manhattan Community College; a faculty member at CUNY's Graduate Center; and a Kavli Scholar at the Kavli Institute for Theoretical Physics.

Gravitational waves can be thought of like the sound waves emitted after an earthquake, but the source of the "tremors" in space are energetic events like supernovae (exploding stars), binary neutron stars (pairs of burned-out cores left behind when stars explode), or the mergers of black holes and neutron stars.

Although scientists have long known about the existence of gravitational waves, they've never made direct observations but are attempting to do so through experiments on the ground and in space.

Part of the reason why detection is difficult is because the waves interact so weakly with matter but McKernan and his colleagues from CUNY, the Harvard-Smithsonian Center for Astrophysics, the Institute for Advanced Study, and Columbia University, suggest that gravitational waves could have more of an effect on matter than previously thought.

The new model shows that stars with oscillations, vibrations, that match the frequency of gravitational waves passing through them can resonate and absorb a large amount of energy from the ripples.

"It's like if you have a spring that's vibrating at a particular frequency and you hit it at the same frequency, you'll make the oscillation stronger," McKernan said. "The same thing applies with gravitational waves."

If these stars absorb a large pulse of energy, they can be "pumped up" temporarily and made brighter than normal while they discharge the energy over time.

This could provide scientists with another way to detect gravitational waves indirectly.

"You can think of stars as bars on a xylophone, they all have a different natural oscillation frequency," said co-author Saavik Ford, who is a research associate in the Museum's Department of Astrophysics as well as a professor at the Borough of Manhattan Community College, CUNY; a faculty member at CUNY's Graduate Center; and a Kavli Scholar at the Kavli Institute for Theoretical Physics.

"If you have two black holes merging with each other and emitting gravitational waves at a certain frequency, you're only going to hit one of the bars on the xylophone at a time but because the black holes decay as they come closer together, the frequency of the gravitational waves changes and you'll hit a sequence of notes. So you'll likely see the big stars lighting up first followed by smaller and smaller ones."

The work also presents a different way to indirectly detect gravitational waves. From the perspective of a gravitational wave detector on Earth or in space, when a star at the right frequency passes in front of an energetic source such as merging black holes, the detector will see a drop in the intensity of gravitational waves measured."

"In other words, stars, including our own Sun, can eclipse background sources of gravitational waves.

"You usually think of stars as being eclipsed by something, not the other way around," McKernan said.

The researchers will continue to study these predictions and try to determine how long it would take to observe these effects from a telescope or detector.

More Information:
B. McKernan, K.E.S. Ford, B. Kocsis, Z. Haiman. "Stars as resonant absorbers of gravitational waves." Monthly Notices of the Royal Astronomical Society: Letters, 2014 - arxiv.org/abs/1405.1414

Rumours that Gravitational waves have been detected

This detailed map of the Cosmic Microwave Background (CMB) is created from seven years worth of data. 

The colour variations correspond to temperature variations in the young universe: the seeds for stars and galaxies observed today. 

Credit: NASA

Last week the Harvard-Smithsonian Center for Astrophysics (CfA) stated rather nonchalantly that they will be hosting a press conference on Monday, March 17th, to announce a "major discovery."

Without a potential topic for journalists to muse on, this was as melodramatic as it got but then the Guardian posted an article on the subject and the rumours went into overdrive.

The speculation is this: a U.S. team is on the verge of confirming they have detected primordial gravitational waves—ripples in the fabric of spacetime that carry echoes of the big bang nearly 14 billion years ago.

If there is evidence for gravitational waves, it will be a landmark discovery, ultimately changing the face of physics.

Not only are gravitational waves the last untested prediction of Albert Einstein's General Theory of Relativity, but primordial gravitational waves will allow astronomers to glimpse the universe in its infancy.

"It's been called the Holy Grail of cosmology," Hiranya Peiris, a cosmologist from University College London, told reporters.

"It would be a real major, major, major discovery." Any convincing evidence would almost certainly lead to a Nobel prize.

The signal is rumoured to have been found by a telescope known as BICEP (Background Imaging of Cosmic Extragalactic Polarization), which scans the sky from the south pole, looking for a subtle effect in the Cosmic Microwave Background (CMB): the radiation released 380,000 years after the big bang when space became transparent to light and photons were allowed to travel freely across the universe.

The South Pole Telescope (left) and BICEP (right). Credit: Dana Hrubes

While the CMB has been mapped in exquisite detail, astronomers think that hidden within the map is a second fingerprint, which would reveal gravitational waves.

Its radiation was scattered toward us from the universe's earliest atoms, similar to the way blue light is scattered toward us from the atoms in the sky and just as the sky is slightly polarized, the waves have a preferred orientation, so is the CMB (on the level of a few percent).

Cosmologists are digging through the data, searching for a subtle twist in the polarized light, known as B-modes.

If a gravitational wave moves through the fabric of spacetime, it will squeeze spacetime in one direction (the universe will look a little hotter) and stretch it in another (the universe will look a little cooler).

The photons will scatter with a preferred direction, leaving a slightly polarized imprint on the CMB, due to the passing gravitational wave.

Andrew Jaffe

"If a detection has been made, it is extraordinarily exciting," Andrew Jaffe, a cosmologist from Imperial College, London, told reporters.

"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 only 10-34 seconds old."