Jun 29 2023

Ripples in Spacetime

Published by under Astronomy
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It’s always exciting when a scientific institution announces that they are going to make an announcement. Earlier this week we were told that there was going to be a major announcement today (June 29th) regarding a gravitational wave discovery. The goal of the pre-announcement is to generate buzz and media attention, although I almost always find the reveal to be disappointing. I guess we are too programmed by movie plotlines where such reveals are truly earthshattering. So I have learned to moderate my expectations (a generally good strategy to avoid disappointment).

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) team released five papers late last night in the Astrophysical Journal Letters – I think my moderated expectations were pretty much on target. This is awesomely cool science, but didn’t shatter my world. I can see why the scientists were so excited, however. This was the culmination of 15 years of investigation. The bottom line discovery is that spacetime is constantly rippling, in line with Einstein’s predictions of General Relativity. Let’s get into the details.

Gravitational wave astronomy is a new window onto the universe. Most of the recent news has been made by the Laser Interferometer Gravitational-wave Observatory (LIGO). This is a large instrument, with two powerful lasers at right angles to each other, with each arm about 4 km long firing through a vacuum pipeline. Where the lasers cross they create an interference pattern. The slightest disturbance in the lasers can change the interference, and therefore it is a very sensitive detector. The primary challenge is isolating LIGO from background noise and filtering it out. What is left is a signal produced by gravitational waves, ripples in spacetime created by massive gravitational events. LIGO is able to detect high frequency gravitational waves formed by the collision of black holes and/or neutron stars with each other.

NANOGrav uses a completely different method to detect gravitational waves. They rely on pulsars, which are rapidly rotating neutron stars that produce beams of radio waves, and are therefore detectable by radio telescopes. The fastest spinning pulsars have periods of just milliseconds, and that period is extremely precise and reliable, as you might imagine, often compared to atomic-clock precision. If, however, there were ripples in the very fabric of space, that could cause these millisecond pulsar intervals to flutter, and that’s exactly what the NANOGrav experiments have been designed to detect.

Unlike LIGO, which detects high frequency gravitational waves from sudden catastrophic events, NANOGrav is optimal for detecting low frequency waves. But this low frequency means that years of observational time is required in order to gather enough data to make sure there is an actual signal in the noise. That is why it has taken 15 years to publish these findings. What the various teams have found is that there is a constant background gravitational wave ripple in the fabric of space. These waves are moving in all directions. What can be causing them?

The primary contender is colliding galaxies with supermassive black holes at their centers. Supermassive black holes have millions to billions of solar masses, where as stellar mass black holes (like the ones detected by LIGO) have masses in the tens to hundreds of stellar masses. Billions of stellar masses generates a lot of gravity. What happens when two such galaxies collide is that their supermassive black holes with orbit each other, slowly spiraling in over millions of years. When their orbits are at a certain range of frequency, that creates gravitational waves at a frequency detectable by NANOGrav. The background waves being reported match the waves that would theoretically be made by closely orbiting supermassive black holes.

The astronomers are not ready to declare that this is the cause of the waves they detected, just the most likely hypothesis for now. But it does sound like a very plausible hypothesis. Orbiting supermassive black holes are rare events, but the universe is a really big place, so “rare” events are happening all the time. At any moment, therefore, there are likely ripples passing over us from many such orbiting supermassive black holes.

This is certainly an interesting finding. It’s more interesting, I think, for what it says about gravitational wave astronomy and its potential for the future. These papers validate the NANOGrav approach and show it can make major discoveries about the universe. Since NANOGrav depends on powerful radio telescopes, it would benefit from larger telescopes and telescope arrays. This is a good complement to LIGO, as the two approaches are looking for different frequency gravitational waves. NANOGrav extends this new approach to astronomy, a new window on the universe, and its very likely there will be many more interesting discoveries in the future.

This finding itself, about the background ripples likely from orbiting supermassive black holes, is interesting but doesn’t really change what we already thought about the universe. It mainly confirms the predictions of General Relativity.  I’m not sure it was worth a pre-announcement announcement, but still interesting.

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