Gravitational Waves

By John McCormick

INFLATION STRIKES EARLY UNIVERSE; gravitational waves detected.

Dateline, 380,023 A.B.B. (After Big Bang), April 1, 2:53 a.m. A spindizzy appears.

Although it may seem like a big yawn, the discovery of evidence proving the existence of gravitational waves is a major advance in physics because it also confirms the Big Bang and inflation theory. This is thought to be a truly Nobel-worthy result although it still remains for peer review to confirm the findings. Gravitational waves are ripples in the space-time continuum.

After two full years of flexing its BICEP2 (Background Imaging of Cosmic Extragalactic Polarization) near the South Pole, The Harvard-Smithsonian Center for Astrophysics (CfA) may have finally vindicated one of the few remaining unproven predictions made by Einstein’s General Theory of Relativity by detecting proof of gravitational waves generated in the very early universe. (See below for a technical description of BICEP.)

There are also “gravity” waves but those are unrelated and part of fluid dynamics.

What this telescope does is measure extremely tiny differences in temperature and look for polarized radiation that would have been caused by gravitational waves in the very early universe.

Although the discovery supports Einstein’s theory of gravity, one thing Einstein might not have been quite so happy about is that the results also strongly support the belief that gravitational waves are quantitized—that is, they possess quantum mechanical features.

While the data produced over two years by BICEP2 have just recently been published (March, 2013) and must be subject to peer review, there have been other recent sky surveys that produced consistent results. Although the other instruments were not capable of sufficiently detailed measurements to actually “prove” the existence of gravitational waves, they could have disproven their existence which they completely failed to do. In fact, there is no real evidence against the existence of gravitational waves.

The new CfA data is impressive enough that there is already talk about this being a Nobel-worthy discovery.

The reason this accomplishment is so important isn’t because it is one more confirmation of General Relativity, but because it offers proof that gravitational waves exist. That result strongly supports inflation theory and opens up the possibility of being able to use gravitational waves themselves as a research tool comparable to electromagnetic (X-ray, infrared, and visible light) waves.

Inflation Theory

Not to be confused with the mere expansion of the universe, which has been happening since long before the end of the first second after the Big Bang, cosmic inflation occurred only during the inflationary “epoch” that ran from 10 to the minus 36th (10^-36) of a second after the Big Bang for an extended period lasting until about 10^-32 of a second A.B.B.

Greatly simplified, inflation was proposed to explain why the universe essentially looks pretty much the same no matter where we look at cosmic background radiation or any other measure—even, and here’s the important bit, in the newly seen portions which were previously invisible to due to light speed limitations.

The line of reasoning which led up to the present situation goes like this: we are in a universe that has been expanding at a slightly accelerating pace since the first second of existence (actually, since about 10^-30 second after the Big Bang). That creates an horizon, an event horizon if you will, limiting what we can observe based on speed of light limits.

In other words, what portion of the universe we can presently observe depends on where we are and how long light has been moving toward us from distant regions. If the universe always expanded at less than the speed of light we could easily see all of it because we were all (based on the Big Bang Theory) in a tiny space to begin with, so we should theoretically have access to light and radio signals from the very edge of the expanding universe of which we are a part.

Sounds obvious enough, but the fact is that we can’t see all of the universe yet. We know that because with the passage of time we see more and more.

Now if the light “horizon” is expanding faster than the universe, that is, the physical expansion is moving less than the speed of light (which it must, according to Einstein and all the evidence), and we are constantly seeing more of the universe, there must be something wrong with our basic understanding of what happened during the Big Bang.

The theory of inflation was proposed by Alan Guth of MIT to explain this and a critical feature of the newly detectable parts of the universe constantly becoming visible. The discovery of gravity waves of the type detected at BICEP2 confirms the inflation theory.

The other feature of the newly appearing parts of the universe involves their appearance. As new parts are revealed, we see that they are remarkably like the parts we already know regarding cosmic background radiation levels and space-time curvature. In scientific terms, side-by-side with the known universe, they are both homogeneous and isotropic.

The problem arises when you try to explain why regions that could never have had any physical communication with the observable universe are so similar to it, especially in terms of heat distribution. Those regions were, until they came into our horizon-view, totally isolated if the universe was always simply expanding and if the speed of light isn’t violated. The explanation of this concern is a bit complex and beyond the scope of this article, but it is widely accepted by the scientific community.

That leaves us with what can only be seen as a remarkable consistency.

If, however, there was a time when, before physical laws applied, the universe “inflated” much, much, much faster than the speed of light, that would explain it. In other words, the universe is uniform in appearance because at one time in the first 10^-30 of a second of its existence, the universe was all within an area that was less than the limit bounded by the speed of light.

What if, after the beginning of everything, but before the universe began expanding at today’s gravitationally linked acceleration, there was a brief period when the universe (Lemaitre’s Primeval Atom) expanded much faster than the speed of light?

For example, what if it expanded from the size of an atom to the size of a beach ball in one 10^-35 of a second? That is much faster than even the speed of light.

That, in a nutshell, was the theory put forth in 1980 by Alan Guth and simultaneously by some others. (Bearing in mind what Einstein said about the impossibility of proving simultaneous events.)

Georges Lemaitre was a Belgian Catholic Priest and astronomer who was the first to propose the idea of an expanding universe (not Hubble). He was a professor of physics and actually published his version of the Big Bang, as well as the first estimate of what is now called Hubble’s Constant several years before Hubble.

This is yet another in a long string of important discoveries and inventions attributed to the wrong person, right up there with Marconi being credited for decades with the invention of the radio when he merely “assembled” a radio which was actually invented by the founder of the 20th century, Nikola Tesla, at least according to the U.S. PTO and the nine Supremes.

Interestingly, Georges Lemaitre’s doctoral advisor was Harlow Shapley, the astronomer who used Cepheid variable stars to measure the size of the universe because of their uniform brightness. He was the first to show that the Milky Way Galaxy is much larger than previously thought.

It is instructive to occasionally look back and realize just how recently the world was seen as a very different place. There are people alive today who were probably taught that all those pesky nebula were part of the Milky Way, and not separate spiral galaxies far outside our local galaxy, an idea actually supported by Shapley but soon disproved when Hubble found Cepheid variables in those “nebulae” and was thereby able to calculate their actual locations.

I mention Shapley in part because he was the head of the Harvard Observatory from 1921 through the early 1950s and played a part in establishing the Center for Astrophysics at Harvard/Cambridge.

Science can be very incestuous at times.

Also, a strange priest even for one taught by the Jesuits (the Pope’s army), Georges Lemaitre also served as a Belgian artillery officer. Perhaps not so strange—he was obviously used to Big Bangs.

Gravitational Waves

Although further confirmation of General Relativity is certainly welcome, gravitational waves aren’t just something for mathematicians and theoretical physicists to cheer about. It is hoped that they can actually be used the way electromagnetic waves (light, radio, etc.) are employed in traditional telescopes. But unlike traditional instruments, gravitational wave detectors/telescopes may be able to probe mysterious objects and regions such as black holes, which can’t be studied using the electromagnetic spectra because light can’t penetrate or escape from them.

They may also be used to look even closer at time-zero, the period between the start of the Big Bang and the first time we can see.

You may recall that General Relativity explained universal (we think) gravitation by describing it as a feature of the space-time continuum—a property of space itself, rather than any type of force. This is illustrated in the most common way by using the three-dimensional analogy of a stretched sheet of rubber with some heavy weight in the middle. This obviously causes the “sheet” of space-time to have a deep, circular depression like a funnel in the sheet.

If you roll a ball along an edge of the sheet away from the weight, what you will see is a straight path followed by the motion of the ball—in other words it travels in a straight line, just what we see in reality. A result fully in agreement with Newton’s laws.

But if you roll a ball across the sheet again, only this time passing near the large weight, the path will curve toward the body not because of some “force” conveyed by a graviton particle, but simply because space itself is bent by large masses.

If it is moving the right speed and passes the right distance from the weight, the ball will “orbit” the object.

This is a little difficult to describe in words because it sounds like nonsense, but is simple in math—a straight line isn’t “straight” near a mass, but it is still the shortest distance between two points, a space-time geodesic.

That may seem like a very abstract concept but you encounter it all the time. Just look at a globe—aircraft flight paths and ship courses are curves—the shortest distance between two points (without penetrating the Earth’s crust) is a portion of a great circle. Coordinates on Earth are not at right angles because they are imposed on the surface of a sphere.

Curvilinear is the description applied to this kind of spatial framework. In Newtonian physics, space was viewed as a coordinate system where three axes (x, y, and z) were always at right angles each to both of the others. In Einstein’s space-time structure, the region surrounding each point in space near any massive (having mass, not just big masses) object will have coordinates at varying, non perpendicular angles to each other. In other words, space is not orthogonal.

That is gravity according to the General Theory of Relativity and a lot of you know all about that theory. But did you ever think what would happen if very large masses moved very quickly? Einstein's theory also predicts that such events would result in gravitational waves, just as waves in water (gravity waves) are created by a moving boat.

The geodesic theory of gravity, i.e., that it isn’t a force but the structure of space, is widely accepted in science because predictions made using it have been tested and found accurate over and over.

One test is gravitational lensing—stars seem to shift position when near the sun—tested and proven.

Another test is to see if we can detect a redshift in light (or other electromagnetic frequency) spectra due to gravity. Proven.

The theory also explains an otherwise unexplained shift in the perihelion of planetary orbits—most noticeable in Mercury. Proven.

All proven for the General Theory. You will probably be surprised to learn there are literally dozens of other space-time theories still attempting to explain or define gravity some different way. Some also pass the above classic tests, but General Relativity is the simplest and is therefore generally accepted.

In science, the simplest theory explaining all the facts is generally accepted as true. For example, does the Earth really orbit the Sun? You can create a mathematical model which accounts for planetary motion based on an Earth-centric rather than a heliocentric solar system, but the model where the Earth orbits the Sun is much simpler and, hence, generally accepted.

Some are proposed either as an ego trip simply to best Einstein. More often, they are proposed to make it possible to explain quantum gravity, or form a unified field theory. Both of which seem impossible under Einstein’s laws.

Gravitational waves are not possible in Newtonian physics where there is no upper limit to possible velocities. They are predicted by the General Theory of Relativity. Proven (but only this March).

Gravitational waves were mostly created in significant quantities and magnitude to first be detected today during a small period of time. The waves are tensor fluctuations in the space-time continuum; therefore, they impose a specific polarization signature on the cosmic microwave background not caused by other phenomena.

But they are theoretically (now far less theoretically) created by a number of astronomical events of much smaller magnitude.

There are two kinds of polarization detected by instruments: E-mode and B-mode polarization. Both scalar and tensor waves can generate E-mode polarization that is symmetric. If you draw it on a piece of paper (dark enough to show through) and flip the paper over, the polarization pattern will still look exactly the same.

B-mode polarization can only be created by a tensor field that, in turn, defines a gravitational wave. A B-mode pattern, if drawn on paper, would not be the same if viewed in a mirror or turned over. In other words, B-mode polarization is “handed,” which makes it easy to distinguish between the signals generated by other processes and those due to gravitational waves. The BICEP2 instrument has measured B-mode radiation in the cosmic background radiation.

Flexing BICEPs

BICEP (Background Imaging of Cosmic Extragalactic Polarization) measures differences in temperature on the order of a millionth of a degree or less.

BICEP2 (the structure on the right in the photo below) measures the cosmic microwave background temperature with a precision never before imagined, let alone achieved and over a wider area than BICEP1, enabling scientists to actually produce a map. The optics of BICEP2 are the same as those in BICEP1. The difference is just in the detection instrument package.

Far from being one of the gigantic scientific instruments we are used to seeing these days, the BICEP telescope itself is only 23 cm—a nine-inch electromagnetic signal gathering scope. The BICEP1 telescope gave consistent results, but covered too narrow a region of the sky to really measure the twists in the gravitational waves. The observatory is located at the Amundsen-Scott South Pole Station.

Placing the telescope in a cold, isolated location is part of the strategy because BICEP2—an upgrade of BICEP1—is, in essence, just a very fancy thermometer. The detector is known as a transition edge sensor bolometer that uses 512 sensors tuned to the 150 GHz. frequency.

BICEP1 had far fewer sensors, but both experiments were designed to measure the polarization of the signals. The first version did so but the field of view was too small for decisive measurements.

BICEP3 will use 2,560 detectors and come online next year. A spokesperson at CfA told “Perihelion” the scientists are working frantically to get the newest instrument package ready for installation.

Gravity Telescopes

Now that Gravitational waves have been detected, they will also exist from less dramatic events than the Big Bang. This gives a big boost for scientists to build an observatory capable of seeing much smaller waves.

The proposed Laser Interferometer Space Antenna (LISA) instrument was a joint project of NASA and the European Space Agency, but NASA budget cuts ended the U.S. participation in an important scientific project. Now eLISA is a project of the eLIST consortium and the ESA.

In terms of sheer size, this instrument will make the CERN collider look like a tiny blip on the scale of scientific instruments. It will consist of three spacecraft in a heliocentric orbit, spaced one million kilometers apart (LISA would have been five times larger giving much better resolution).

Rather than measuring byproducts of the Big Bang’s gravitational waves, eLISA/LISA will measure gravitational waves directly, allowing astrophysicists to observe previously unmeasured objects, including black holes.

Unlike Keck and BICEP, LISA isn't designed to demonstrate the existence of gravitational waves but to use them as an optical telescope uses light.

The telescope isn’t scheduled to be operational until sometime between 2028 and 2034. There are precise schedules published, but that far out government projects and schedules tend to be exceedingly vague. By then the U.S. might even be interested in investing in science again, permitting the better version to be built.

A trial version/mission is scheduled to launch next year which should prove whether the full LISA mission is practical. Of course, by 2028 we will have seen several more computer generations as well as ever more sensitive detection sensors, so if LISA is shown to be vaguely practical in 2015 it should prove extremely useful and productive when actually constructed and launched 15 years later.

Gravitational wave telescopes and as well as the proof of the existence of gravitational waves open up an entirely new branch of physics and make available exciting new vistas for astrophysicists to explore. 

Further Reading

Detection Of B-mode Polarization at Degree Angular Scales.
BICEP Home Page.
South Pole Telescopes.
Non-Standard Models and the Science of Cosmology.

John McCormick is a trained physicist, science/technology journalist, and widely-published author with more than 17,000 bylines to his credit. He is a member of The National Press Club and the AAAS. His full bibliography can be accessed online.