Neutron stars scream in waves of spacetime when they die, and astronomers have outlined a plan to use their gravitational agony to trace the history of the universe. Join us as we explore how to turn their pain into our cosmological profit.
Cosmologists are obsessed with standards. The reason for this obsession rests in their laborious attempts to measure extreme distances in our universe. Look at a random star or galaxy. How far away is it? Is it closer or farther than a star or galaxy next to it? What if one is brighter or dimmer than the other?
This is a pretty hopeless situation, unless the cosmos is scattered with standard things – objects with known properties. Imagine if 100-watt lightbulbs or meter sticks littered the universe. If we could see those lightbulbs or meter sticks, we could compare how they look to us here on Earth to what we know they look like up close and personal. If we see a lightbulb in the universe, and know that it’s supposed to be the same brightness as a standard 100-watt bulb, then we can do some trigonometry to knock out the distance to that bulb. Same for the stick: if we see a random stick floating around, and know that it’s supposed to be exactly one meter long, we can compare its length in our field of view and math out the distance to it.
Of course lightbulbs and meter sticks would make for lousy cosmological probes, because they’re dim and small. For serious work we need bright things, big things, and common things. And there are precious few of these standards in the universe: Type 1a supernova serve as “standard candles” and baryon acoustic oscillations (a remnant baked into the distribution of galaxies leftover from the early universe, and the subject of another article) can serve as a “standard ruler”.
But we’re going to need more than candles and sticks to get us out of the current cosmological conundrum we find ourselves in.
We live in an expanding universe. Every day, galaxies get further away from each other (on average; there can still be “small scale” collisions and groupings). And the expansion rate of our universe has changed over the past 13.8 billion years of cosmic history. The universe is made of a bunch of different characters: radiation, stars, gas, weird things like neutrinos, weirder things like dark matter, and weirdest things like dark energy. As each of these components turns on, turns off, begins to dominate, or stops dominating, the expansion rate of the universe in turn shifts.
Way back in the good old days, matter used to be the boss of the universe. So as the universe expanded, that expansion slowed down from the constant gravitational tugging of all that matter. But then the matter got too spread out, too thin, and too feeble to control the cosmos.
About five billion years ago, dark energy took control, reversing the slight deceleration of the universe’s expansion and pushing the petal to the metal, causing the expansion of the universe to not just continue, but to accelerate. Dark energy – whatever that is – continues its sinister dominance of the cosmos to the present day.
It’s critically important to measure the expansion rate of the universe right now – since the expansion rate is tied to the contents of the universe, measuring the expansion rate today tells us who the major cosmological players are and their relative importance. We can measure today’s expansion rate, known as the Hubble constant, a lot of ways, like with sticks and candles.
And herein lies a surprising tension. Measurements of the Hubble constant from the nearby universe using things like supernova give one particular value. But measurements of the early universe using the cosmic microwave background also lead to constraints on today’s Hubble constant, and these measurements doesn’t quite agree with each other.
A sticky problem: two independent methods of measuring the same number lead to different results. It could be a sign of brand new physics or just poorly-understood observations. But whatever the case, while some cosmologists look at this situation as a challenge, others look at it as an opportunity. What we need are more measurements, and especially ones that are totally independent from the existing ones. We have standard rulers and standard candles, so how about…standard sirens.
Sure, why not.
The cacophonous gravitational waves blasting from the final moments of the collisions of two neutron stars carry juicy cosmological information. Since we understand their physics very well, we can study the ultra-precise structure of the gravitational waves to know how loud (in gravity, not in sound, but you’ll just have to roll with the metaphor) they were screaming when they collided. Then we can compare that to how loud they sound here on Earth, and voila: a distance.
This technique has already yielded a (relatively rough) measurement of the Hubble constant from the one and only observed neutron star merger.
But that shouldn’t be the last neutron star death-scream we hear. Over the coming years we expect (hope?) to catch dozens more. And with every collision we can pin down a reliable distance to the fiery event and measure the expansion history of the universe since their neutrony doom, providing a completely different track to revealing the value of Hubble’s constant.
Cosmologists at the University of Chicago predicted that within five years, the technique of standard sirens will provide measurements competitive with existing methods. But when it comes to the great cosmological debate of the 21st century, the question remains: will standard sirens be the deciding factor, or only deepen the mystery?
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