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The oldest light we can see comes from when the universe was less than 400,000 years old, so how can we study those first few moments of history?

Hosted by: Caitlin Hofmeister

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Sources:

https://www.ligo.caltech.edu/page/what-are-gw
https://www.universetoday.com/139032/how-to-listen-to-the-background-hum-of-gravitational-waves-from-all-the-black-holes-colliding-into-each-other/
https://iopscience.iop.org/article/10.1088/1742-6596/840/1/012030/pdf
https://www.ligo.org/science/GW-Stochastic.php
http://science.sciencemag.org/content/sci/328/5981/989.full.pdf
https://arxiv.org/pdf/1801.04268.pdf
https://www.cfa.harvard.edu/~cbischoff/cmb/
http://discovermagazine.com/2016/sept/14-revisiting-primordial-gravity-waves
https://www.nature.com/news/polarization-detected-in-big-bang-s-echo-1.13441
https://arxiv.org/pdf/astro-ph/9803150.pdf

https://arxiv.org/pdf/1811.08797.pdf

Images:

https://en.wikipedia.org/wiki/File:Spacetime_lattice_analogy.svg
https://svs.gsfc.nasa.gov/12949
https://www.youtube.com/watch?v=i1-3eClv_TY
https://commons.wikimedia.org/wiki/File:Ilc_9yr_moll4096.png
https://en.wikipedia.org/wiki/File:Simplified_diagram_of_an_Advanced_LIGO_detector.png

https://svs.gsfc.nasa.gov/10127
https://en.wikipedia.org/wiki/File:LISA-waves.jpg
https://svs.gsfc.nasa.gov/30093
https://svs.gsfc.nasa.gov/10426
https://svs.gsfc.nasa.gov/12656
[♪ INTRO].

Telescopes, you may have heard, are time machines. Because light has a speed limit, the deeper into space we look, the older the signal we receive.

The oldest light we can see is called the Cosmic Microwave Background, and it comes from when the universe was less than 400,000 years old. And while that’s great and all, it also means there are 400 thousand years of history we can’t study using traditional methods. That’s why astronomers are so interested in finding techniques that don’t rely on light.

And luckily for them, and us, there are some other waves out there that could reveal the universe when it was a teeny tiny fraction of a second old. I’m talking about gravitational waves. Over a century ago, Albert Einstein taught us that mass deforms the fabric of spacetime, kind of like how a bowling ball deforms a trampoline.

But he also predicted that accelerating mass would cause space itself to ripple, like the surface of a pond. And back in 2015, we directly detected these gravitational waves for the first time. That was thanks to a pair of black holes spiraling inward and merging with one another.

But technically, lots of things in space can cause gravitational waves. And if we think of spacetime like the surface of a lake, all of these astronomical events are like raindrops, whose gravitational waves interfere with one another and generate a kind of noise. Theoretically, we could someday pick apart that noise to study specific events.

But what’s maybe even more interesting is that, beneath that noise, space is actually filled with evidence of other, older gravitational waves. And those waves could teach us about the birth of the universe itself. Waves from way back then are called primordial gravitational waves, and there are a few proposed sources for them.

According to many cosmologists, some were generated by the formation and merger of still-hypothetical primordial black holes. These objects would act like regular black holes, but would be less massive, and may have sprung up from pockets of super dense matter in the very early universe. Other primordial gravitational waves could have been generated by the formation of various particles as the universe cooled down.

The ultimate primordial waves though, weren’t caused by stuff in space, they were made by space itself. They come from a hypothetical period in the universe’s history called inflation. It’s the time a tiny fraction of a second after the Big Bang, around 10-32 to 10-36 seconds, when most cosmologists believe the universe expanded way faster than the speed of light.

For the record, this wouldn’t break the law that says nothing can travel faster than the speed of light, because that law only applies to matter in space, not to space itself. Regardless, inflation still isn’t set in stone. There are definitely alternative interpretations for what could have happened back then.

Gravitational waves are predicted in these alternative hypotheses, too, but detecting primordial waves will hopefully give cosmologists the data they need to pin down what actually happened. For example, they could use the amplitude of the waves to help define how fast everything expanded, the energy involved in inflation, and exactly when and for how long it happened. And through other methods, they could learn how consistent that inflation was across the whole universe.

Of course, before we can figure out any of that, we have to actually detect these waves. And we do have a few options. First, there’s the indirect method of detection, which comes from looking at the Cosmic Microwave Background.

According to the math, gravitational waves older than the CMB would have influenced what it looks like. Specifically, they would have caused a certain spiral pattern in the light that cosmologists call B-mode polarization. We can already detect a different kind of polarization in the CMB, called E-mode, and scientists are investigating the B-mode kind.

But it’s hard because it’s a way weaker effect, and these signals can also come from things like dust in the Milky Way. The other option, of course, is to just try to directly detect primordial gravitational waves, using equipment similar to what we’ve used to detect the waves from black hole mergers. Right now, to detect those events, we mainly use interferometers like LIGO, which send laser pulses down two perpendicular arms.

If a gravitational wave passes through the system, it will compress or stretch things, meaning one laser beam will have to travel farther than the other. Unfortunately, none of our current interferometers is sensitive enough to detect primordial waves, but there are future projects in the works. The main one is LISA, which will work roughly the same way as LIGO, except in space.

It’ll consist of three spacecraft, arranged in a triangle and separated by millions of kilometers, and it’s scheduled to launch in 2034. The other direct detection method uses dense, spinning objects called pulsars. They shoot out beams of radiation as they rotate, which can hit Earth at really regular intervals.

But if a gravitational wave passed through the space between the pulsar and Earth, that interval would change. And astronomers would be able to use details about the wave’s signal to figure out if they came from primordial or recent sources. Still, figuring out what “normal” means is complicated, because even if pulsars are known for being predictable, there are still other factors that can affect how fast they rotate.

And it’s going to take time for scientists to pin down a model that’s good enough to use pulsars effectively. But once we find those elusive primordial waves, it will mean big things for astronomy. We’ll be able to figure out more about inflation, and see back further than we ever have before.

And with more research, we’re getting closer and closer to understanding the moment our universe’s story began. Thanks for watching this episode of SciShow Space! If you want to learn more about other tools we could use to study the universe, you can watch our episode about the Cosmic Neutrino Background after this. [♪ OUTRO].