Gravitational Waves 2

This is the second part of a two-part post on gravitational waves. For the first part, see here.

The previous post introduces gravitational waves, and discusses early attempts at their detection, such as LIGO. Despite LIGO's failure to detect these waves, new instruments promise to increase precision, allowing us to find weaker gravitational waves, and other indirect methods have yielded results. Overall, these techniques will give us new methods of observing our Universe.

LIGO, as well as other early laser interferometer observatories such as VIRGO (a similar detector in Italy), are sometimes known as the "first generation" of gravitational wave detectors. During LIGO's operation, upgrades led to modest increases in sensitivity. However, after the temporary cessation of operations in 2010, more major upgrades were made to LIGO including heavier mirrors and more powerful lasers, which will increase sensitivity and reduce background noise caused by thermal energy. The new Advanced LIGO began operation in 2016, and should have a range of hundreds of millions of light years, ten times that of the original design (see diagram below). These upgraded observatories were the "second generation" of detectors.

The original LIGO could detect gravitational wave sources only within our Local supercluster and its neighbors (small gray sphere), but Advanced LIGO was able to scour an volume of space 1000 times as large for gravitational wave signals (the entire scope of the figure above).

The above diagram shows the actual and estimated sensitivities for different gravitational wave detectors, including LIGO, VIRGO, and their respective upgrades. The x-axis of the graph is the frequency of the gravitational wave (gravitational waves have different frequencies in the same way that electromagnetic waves do) and the y-axis indicates the intensity of the waves. The detectors exhibit different sensitivities to different frequencies; curves that dip lower indicate better detectors. The Einstein GW Telescope is a proposed "third-generation" laser interferometer concept, still in design phase. This design would have the facility be underground to reduce seismic noise and cryogenically cooled to prevent thermal vibrations from altering the distance between mirrors.

Another "third-generation" design concept which would theoretically yield numerous detections is the Laser Interferometer Space Antenna (LISA), a spaced-based model.

LISA would consist of three separate spacecraft, which would create a equilateral triangle of side length 5 million kilometers (3.1 million miles). The above diagram is an artist's conception. This triangle would trail the Earth in heliocentric orbit, and would be very sensitive to different frequencies of gravitational waves than ground-based detectors like LIGO.

The above figure shows that LISA would detect much longer wavelengths than Advanced LIGO (due to LISA's enormous arms). Advanced LIGO could only discover very high frequency oscillations, such as neutron stars rotating very close to each other just before colliding. Such systems are rare and short-lived, since neutron star systems contract and ultimately collide. However, LISA could detect more slowly orbiting binary systems, long before their final collision. These are very common, and many are already known through other means of observation, guaranteeing that LISA would find many sources if it functions correctly.

There are unfortunately no definite plans for launching LISA, but a small test mission, known as LISA Pathfinder, launched in 2015. This small probe contained a tiny interferometer meant to test the LISA concept in space and evaluate the proposal's feasibility.

Ultimately, the most important goal of gravitational-wave observatories is to peer farther into the early universe than could be possible with telescopes measuring electromagnetic radiation. Using ordinary visual telescopes (of sufficient power), we can view objects billions of light years away (seeing them as they were billions of years ago, since it takes light a year to travel each light-year). However, there is a fundamental limit to how far these telescopes can see. Before 380,000 years after the Big Bang (or about 13.8 billion years ago), the temperature of the Universe was too high for electrons to combine with atomic nuclei into atoms, and, since electrons scatter electromagnetic radiation, the Universe was opaque. Thus the "oldest" light in the Universe is from 380,000 years after the Big Bang; it is called the Cosmic Microwave Background (CMB), and traditional telescopes cannot see farther. However, gravitational wave astronomy has the potential to receive signals from earlier periods and study them directly, leading to a greater understanding of the Big Bang.

For an update on recent developments in the detection of gravitational waves, see here!

Sources: https://www.advancedligo.mit.edu/summary.html, http://www.ligo.caltech.edu/docs/G/G080303-00.pdf, http://www.et-gw.eu/, http://www.physik.hu-berlin.de/qom/research/freqref/lisa, http://lisa.nasa.gov/, http://cosmology.berkeley.edu/~yuki/CMBpol/CMBpol.htm, http://www.theguardian.com/science/2014/mar/17/primordial-gravitational-wave-discovery-physics-bicep, http://www.nytimes.com/2014/03/25/science/space/ripples-from-the-big-bang.html?_r=0, "An Ear to the Big Bang" from The Scientific American October 2013 issue