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  • [♪ 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 whatnormalmeans 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]

[♪ INTRO]

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B2 中高級

我們如何研究宇宙的第一個納秒? (How We Could Study the First Nanoseconds of the Universe)

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    林宜悉 發佈於 2021 年 01 月 14 日
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