Did LIGO Just Discover Two Fundamentally Different Types Of Neutron Star Mergers?
“The first neutron star-neutron star merger ever directly observed was seen in both gravitational waves and in various forms of light, giving us a window into the nature of short gamma ray bursts, kilonovae, and the origin of the heaviest elements of all. The second one, however, had no robustly confirmed electromagnetic counterpart at all. The only major physical differences were the combined mass (2.74 vs. 3.4 solar masses), the initial object formed (neutron star vs. black hole), and the distance to the event (130 vs. 518 million light-years).
It’s possible that there really was an electromagnetic counterpart, and we simply weren’t able to see it. However, it’s also possible that binary neutron star mergers that directly lead to a black hole don’t produce electromagnetic signatures or enriched, heavy elements at all. It’s possible that this binary neutron star system, the most massive one ever discovered to date, represents a fundamentally different class of objects than have ever been seen before. This incredible idea should get put to the test over the next few years, as gravitational wave detectors continue to find more and more of these mergers. If there are two different classes of neutron star mergers, LIGO and Virgo will lead us to that conclusion, but we have to wait for the scientific data to know for sure.”
Neutron stars, when they merge, can produce gamma ray bursts, ejecta, the heaviest elements of all, and electromagnetic afterglows that cover nearly the full spectrum of where light can exist. We saw this with a gravitational wave + gamma ray signal that occurred on August 17, 2017, leading to a revolutionary understanding of kilonovae. But our second merging binary neutron star system, which was seen in gravitational waves on April 25, 2019, displayed no such signal at all.
It might be a failure of detection, or it might be due to some fundamental differences between these different events. My bet’s on the latter; here’s what we think might be going on!
The Smallest Galaxies Have Off-Kilter Black Holes, But Astronomers Know Why
“Over 100 dwarf galaxies are now known to possess these black holes, with the first verified one discovered in 2011. However, solely finding radio emissions isn’t enough: active black holes and star-formation bursts can create that signal. Researchers led by Dr. Amy Reines just conducted the first large-scale radio survey looking for black holes in dwarf galaxies. Using the Very Large Array, her team surveyed 111 dwarf galaxies, and found 13 of them that showed evidence for massive black holes. Remarkably, approximately half of the black holes were not located at the galaxy’s centers, but were significantly off-kilter.”
When we examine the supermassive black holes we find in the Universe, they’re pretty much always found at the centers of galaxies. However, these are for black holes of millions-to-billions of solar masses and galaxies comparable in mass (or even greater than that) to the Milky Way. But dwarf galaxies, the majority of galaxies in the Universe, are predicted to have much smaller black holes. The first large survey of these galaxies was just undertaken, revealing a population of dwarf galaxies with black holes.
But half of them are located off-center, rather than at the center! Why is that? Astronomers know, and you can too!
Sorry Science Fans, Discovering A 70-Solar-Mass Black Hole Is Routine, Not Impossible
“Astronomers aren’t perplexed by this object (or similar ones to it) at all, but rather are fascinated with uncovering the details of how they formed and how common they truly are. The mystery isn’t why these objects exist at all, but rather how the Universe makes them in the abundances we observe. We don’t falsely generate excitement by spreading misinformation that diminishes our knowledge and ideas prior to this discovery.
In science, the ultimate rush comes from discovering something that furthers our understanding of the Universe within the context of everything else we know. May we never be tempted to pretend anything else is the case.”
Did you hear about this “impossible” black hole that “perplexes” astronomers and “defies” theory? If you followed the news cycle last week, that’s probably what you’ve heard. But the truth is far more interesting, and includes facts like:
-this is the fourth black hole we’ve found like it, not the first,
-there are two other ways to make black holes that would explain this object in addition to the one way that can’t,
-and that we’ve seen each and every one of the steps necessary to make a black hole like this,
-but that finding this black hole with this particular method really is revolutionary?
As always, the real science is far more interesting than the mangled hype you’ve seen before. This black hole doesn’t defy theory, but sure does teach us a lot. Come get the real story today.
Starts With A Bang #48 – The Event Horizon Telescope
Earlier this year, 2019, the Event Horizon Telescope collaboration revealed the first image that directly showed the existence of an event horizon around a black hole. This image, constructed from many petabytes of data from telescopes observing the same target, simultaneously, from all across the Earth, provided a breathtaking confirmation of Einstein’s relativity in a realm where it had never been tested before. But that’s just one image of one black hole at one particular moment in time, and there’s so much more to come from the Event Horizon Telescope.
This month, we’re so fortunate to sit down with EHT scientist Sara Issaoun, who takes us through the past, present, and future hopes for the Event Horizon Telescope and how it hopes to answer humanity’s biggest questions about black holes.
(Image credit: APEX, IRAM, G. Narayanan, J. McMahon, JCMT/JAC, S. Hostler, D. Harvey, ESO/C. Malin)
Ask Ethan: Can Black Holes And Dark Matter Interact?
“If you do the math, you’ll find that black holes will use both normal matter and dark matter as a food source, but that normal matter will dominate the rate of growth of the black hole, even over long, cosmic timescales. When the Universe is more than a billion times as old as it is today, black holes will still owe more than 99% of their mass to normal matter, and less than 1% to dark matter.
Dark matter is neither a good food source for black holes, nor is it (information-wise) an interesting one. What a black hole gains from eating dark matter is no different than what it gains from shining a flashlight into it. Only the mass/energy content, like you’d get from E = mc2, matters. Black holes and dark matter do interact, but their effects are so small that even ignoring dark matter entirely still gives you a great description of black holes: past, present, and future.”
You might not be able to make a black hole out of dark matter entirely, but once a black hole exists, anything that falls past its event horizon will add to its mass, whether it’s particles, antiparticles, radiation or dark matter. And the longer black holes sit in the galaxy, the more and more dark matter will eventually fall in.
The question isn’t whether dark matter contributes to black holes; it’s how and how much. Let’s give you the answer on this edition of Ask Ethan!
Sorry, Black Holes Aren’t Actually Black
“If you have an astrophysical object that emits radiation, that immediately defies the definition of black: where something is a perfect absorber while itself emitting zero radiation. If you’re emitting anything, you aren’t black, after all.
So it goes for black holes. The most perfectly black object in all the Universe isn’t truly black. Rather, it emits a combination of all the radiation from all the objects that ever fell into it (which will asymptote to, but never reach, zero) along with the ultra-low-temperature but always-present Hawking radiation.
You might have thought that black holes truly are black, but they aren’t. Along with the ideas that black holes suck everything into them and black holes will someday consume the Universe, they’re the three biggest myths about black holes. Now that you know, you’ll never get fooled again!”
So, you thought you knew all there way to know about black holes? That if you get enough mass together in a small enough volume of space, you create an event horizon: a region from within which nothing can escape, not even light. So how is it, then, that black holes wind up emitting radiation, even long after the last particle of matter to fall into them has ceased?
There are two ways this occurs, and both are completely unavoidable. Black holes aren’t actually black, and this is how we know it.
No, Black Holes Will Never Consume The Universe
“Yes, there will be a very, very small number of stars, planets, asteroids and more that do get consumed by black holes, but it will be less than 0.1% of all the matter presently in the Universe. Even dark matter will remain in the outskirts of galaxies, unable to be eaten by black holes.
You might think that after googols and googols of years, anything still present in a galaxy will eventually be consumed, but don’t forget about Hawking radiation: eventually, all the Universe’s black holes will decay, too. Before any substantial fraction of the remaining galactic matter — normal or dark — can be devoured, every black hole in the Universe will have completely decayed away. If something dear to you does fall into a black hole, don’t despair. Try waiting instead. If you’re clever enough, you’ll not only get its energy back again someday, but most likely its information, too.”
About a month ago, I gave a talk in Hungary at their big international event: Brain Bar, where I spoke about the biggest myths about black holes. One of them is the idea that eventually, if you wait around for long enough, black holes will consume the entire Universe. It makes sense to think that this could happen, since gravity is real, there are close to a billion black holes in our galaxy, objects do randomly collide with one another, and gravitational radiation cause all bound masses to eventually inspiral into one another. But, as it turns out, something else happens first.
The overwhelming majority of matter will never find its way into a black hole, and black holes won’t consume the Universe. Here’s what happens instead.
How Did This Black Hole Get So Big So Fast?
“Recently, a new black hole, J1342+0928, was discovered to originate from 13.1 billion years ago: when the Universe was 690 million years old, just 5% of its current age. It has a mass of 800 million Suns, an exceedingly high figure for such early times. Even if black holes formed from the very first stars, they’d have to accrete matter and grow at the maximum rate possible — the Eddington limit — to reach this size so rapidly. Fortunately, other methods may also grow a supermassive black hole.”
One of the puzzles of how our Universe grew up is how the supermassive black holes we find at the centers of galaxies got so big so fast. We’ve got multiple black holes that come from when the Universe was less than 10% of its current age that are already many hundreds of millions, if not billions, of solar masses in size. How did they get so big so fast? While many hypothesize exotic scenarios like our Universe being born with (primordial) black holes, there is no evidence for such an extraordinary leap. Could conventional astrophysics, and the realistic conditions of our early Universe, actually lead to black holes so massive so early on?
The answer is very likely yes. Come see an extremely favored scenario, with nothing more than conventional astrophysics, that just might get us there.
10 Deep Lessons From Our First Image Of A Black Hole’s Event Horizon
“6. Black holes are dynamic entities, and the radiation emitted from them changes over time. With a reconstructed mass of 6.5 billion solar masses, it takes roughly a day for light to travel across the black hole’s event horizon. This roughly sets the timescale over which we expect to see features change and fluctuate in the radiation observed by the Event Horizon Telescope.
Even with observations that span only a few days, we’ve confirmed that the structure of the emitted radiation changes over time, as predicted. The 2017 data contains four nights of observations. Even glancing at these four images, you can visually see how the first two dates have similar features, and the latter two dates have similar features, but there are definitive changes that are visible — and variable — between the early and late image sets. In other words, the features of the radiation from around M87’s black hole really are changing over time.”
I’ve heard some grumbling over the past day that people are unimpressed with the Event Horizon Telescope collaboration’s big reveal. Maybe the image doesn’t look pretty enough for some people; maybe it doesn’t have the sharpness or level of detail that people are used to from observatories like Hubble.
Well, may I please introduce you to science? If you knew what we’ve actually learned by taking this image, you might change your tune. Read this, and see if you’re not impressed now!