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Tag Archives: gravitational waves

The detection of gravitational waves by the Laser Interferometer Gravitational-wave Observatory (LIGO) opens up a method of observing the universe in a new way, perhaps comparable to the advance that Galileo made in 1609 by turning a telescope on the heavens.

Many awards have been given to Kip Thorne of Caltech, who worked out the theory decades ago, and the people who built LIGO over 40 years until its success in 2015.  The 2017 Nobel Prize in Physics was awarded to Prof. Thorne as well as Barry Barish and Rainer Weiss, half to Weiss for beginning the actual method of the successful observations and one quarter each to the other two.


Credit: Niklas Elmehed/The Nobel Prize Foundation


The October 3, 2017, award of the Nobel Prize was much anticipated and is well deserved.  See, for example, coverage in the following media:
Physics Today
NY Times
The Atlantic provides a thoughtful reflection on the merits of the Nobel prize.


The Laser Interferometer Gravitational Wave Observatory (LIGO) was a topic of NPR’s Morning Edition, August 17, 2016.


Credit: Caltech/MIT/LIGO Lab

Read a full transcript or listen to the broadcast here (5 min 30s).

On June 15, 2016, scientists at LIGO announced they had detected a second pair of black holes merging, at a distance of 1.4 billion light years, releasing the energy equivalent of the mass of the Sun. The consequent ripples in space-time shook the twin detectors at LIGO on December 26, 2015.

This brings the number of confirmed detections by LIGO to two within just four months, giving scientists optimism that more events will follow, enabling quantitative predictions about how frequently these high-energy events occur across space and time.

Read more in this NYT article, including a short video.

An update for Section 13.3f (pp. 352-355) in Pasachoff & Filippenko, The Cosmos, 4th ed:
In what many think is the most important step in astronomy since Galileo first turned a telescope on the heavens in 1609, ripples in space-time known as gravitational waves were detected on September 14, 2015, and reported to the world at large on February 11, 2016. Scientists gathered all over in auditoriums at the National Press Club in Washington, at Caltech, at MIT, in Moscow, and in many other places for the epochal event. “Ladies and gentlemen, we have detected gravitational waves,” said the executive director of the LIGO Laboratory. “We did it!”

The observational existence of gravitational radiation had been established by the changing period of the Hulse-Taylor “double pulsar” at a rate that matched the energy loss expected from the emission of gravitational waves (Section 13.3f, pp. 352-354), but the gravitational waves themselves – distortions of space-time – had not been detected directly until the Advanced LIGO (Laser Interferometer Gravitational Wave Detector) picked up a signal in its engineering run on September 14, 2015.  The signal, which could be heard as an upwards “chirp” in the audio range, resulted from two black holes 1.3 billion light years away, each containing about 30 solar masses, spiraling rapidly into each other and merging to become a single black hole with almost the same total mass but with 3 solar masses of material converted into energy in the form of gravitational waves.

Advanced LIGO has about 3 times the sensitivity of the earlier LIGO, and this event was within that advance of sensitivity. A further gain of 3x is expected with Advanced LIGO in the future.

Scientific papers were published February 11, 2016, in the Physical Review Letters and in the Astrophysical Journal Letters.

A few days later, I [JMP] attended a two-hour evening session at Caltech’s main auditorium to hear the principals of the LIGO project speaking, and I had the feeling that it was just like being at a meeting of the Lincei Academy in Florence in 1610 to hear Galileo speak about his new discoveries with the [optical] telescope.

There are many links to discussions and animations available. Here are some of them:

Popular and academic press:
New York Times coverage (includes movie); also NYT Opinion piece and an NYT Editorial a few days later, justifying scientific inquiry; a congressman’s letter, and sound bites from scientists.
Nature coverage (with explanatory graphics)
Science magazine in depth (requires subscription to access full text)
New Yorker article (with an account of LIGO’s inception and development)
Physics Today comparison of gravitational waves and sound waves
The Wall Street Journal and Michio Kaku’s desciption.

Societies and organizations:
American Physical Society Viewpoint
The Kavli Foundation Scientific spotlight
Caltech (press releasesillustrations, movies and animations)
NSF press release
AAPT resources on gravitational waves
STFC (UK) press release
Cornell Chronicle and media statement
University of Texas Rio Grande Valley press release
ESA congratulations
CSIRO (Australia) news release

LIGO websites:
LIGO labs (Observatories: Livingston | Hanford); Advanced LIGO; LIGO Scientific Collaboration; LIGO Partner Experiments and Collaborations






From ESA press releases, January 31 and February 5, 2015:

New maps from ESA’s Planck satellite uncover the ‘polarized’ light from the early Universe across the entire sky, revealing that the first stars formed much later than previously thought.


Credit: ESA and the Planck Collaboration

Between 2009 and 2013, Planck surveyed the sky to study this ancient light in unprecedented detail. Tiny differences in the background’s temperature trace regions of slightly different density in the early cosmos, representing the seeds of all future structure, the stars and galaxies of today.

Scientists from the Planck collaboration have recently published the results from the analysis of these data in a large number of scientific papers over the past two years, confirming the standard cosmological picture of our Universe with ever greater accuracy.

However, despite earlier reports of a possible detection of gravitational waves in the polarization of the CMB, a joint analysis of data from ESA’s Planck satellite and the ground-based BICEP2 and Keck Array experiments has found no conclusive evidence of primordial gravitational waves.

Links: full ESA press release and another one; Planck mission home; details about the CMB map including hi-res images.

From a Harvard-Smithsonian Center for Astrophysics press release:

Almost 14 billion years ago, the Universe burst into existence in an extraordinary event that initiated the big bang. In the first fleeting fraction of a second, the Universe expanded exponentially, stretching far beyond the view of our best telescopes (see Section 19.5, p. 526).

Researchers announced on March 17, 2014, the first direct evidence for this cosmic inflation. Their data also represent the first images of gravitational waves – ripples in space-time. These waves have been described as the ‘first tremors of the big bang.’ Finally, the data confirm a deep connection between quantum mechanics and general relativity.

Credit: Steffen Richter (Harvard University)

These groundbreaking results came from observations by the BICEP2 telescope (pictured above) of the cosmic microwave background – the faint glow left over from the big bang. Tiny fluctuations in this afterglow provide clues to conditions in the early universe. For example, small differences in temperature across the sky show where parts of the Universe were denser, eventually condensing into galaxies and galactic clusters.

Since the cosmic microwave background is a form of light, it exhibits all the properties of light, including polarization. On Earth, sunlight is scattered by the atmosphere and becomes polarized, which is why polarized sunglasses help reduce glare. In space, the cosmic microwave background was scattered by atoms and electrons and became polarized too.

The researchers hunted for a special type of polarization called ‘B-modes,’ which represents a twisting or ‘curl’ pattern in the polarized orientations of the ancient light. Gravitational waves squeeze space as they travel, and this squeezing produces a distinct pattern in the cosmic microwave background. Gravitational waves have a ‘handedness,’ much like light waves, and can have left- and right-handed polarizations. The swirly B-mode pattern is a unique signature of gravitational waves because of their handedness.

Credit: BICEP2 Collaboration

The figure above shows the actual B-mode pattern observed with the BICEP2 telescope, with the line segments showing the polarization from different spots on the sky. The red and blue shading shows the degree of clockwise and anti-clockwise twisting of this B-mode pattern.

Links: the Harvard-Smithsonian CfA press release including figures, Caltech press release, NY Times article by Dennis Overbye (including a cartoon explaining inflation), Union-Tribune San Diego article (including cartoon of polarization of light), APOD March 18, 2014 shows the observatory at the South Pole, all BICEP2 public pages.