If you took one of history’s top scientists from 100 years ago and dropped them into today’s world, what scientific revelations do you think would shock them the most? Would they be surprised to learn that the stars, which emit almost all of the light we see from the Universe beyond Earth, make up only a tiny fraction of the Universe’s mass? Would they be baffled at the existence of supermassive black holes, the most massive single objects in the Universe? Or would it be dark matter or dark energy that they found most puzzling?
It would be easy to understand their disbelief. After all, science is an empirical endeavor: our understanding of the natural world and Universe is informed primarily by what we observe and measure. It’s hard to fathom that objects or entities that emit no light of their own — that are not themselves directly observable through our telescopes — would somehow make up such a massive, important component of our Universe. And yet, almost every scientist working today has come to the same conclusion: our Universe is mostly dark. Here’s how we learned about it.
On the theoretical side, it’s important to recognize two separate things right from the start:
- theory tells us what to expect given certain conditions,
- but it also only tells us what’s possible in the Universe, not what our assumptions about the Universe’s conditions should be.
When Einstein put forth our modern theory of gravitation — General Relativity — it did something that no other theory did. It not only succeeded everywhere the prior (Newton’s) leading theory did, but it made a novel set of predictions that were distinct from that prior theory. It successfully explained the orbit of Mercury, which was an unsolved problem previously. It accommodated and included the observed facts of time dilation and length contraction. And it made novel predictions about the gravitational bending and shifting of light, which led to concrete observable consequences.
Just a few years after it was proposed, critical tests were performed, confirming the predictions of Einstein’s theory as matching our Universe and rejecting the null (Newtonian) hypothesis.
What Einstein’s General Relativity gives us is a framework for understanding the phenomenon of gravitation in our Universe. It tells us that, dependent on the properties and configuration of the matter and energy in the Universe, spacetime will curve in a particular way. The curvature of that spacetime, in turn, tells us how matter and energy — in all its forms — will move through that spacetime.
From a theoretical viewpoint, this gives us virtually limitless possibilities. You can concoct a Universe with any configuration you like, with any combination of masses and particles of radiation and fluids of various properties that you like, distributed however you choose, and General Relativity will tell you how that spacetime will curve and evolve, and how any components will move through that spacetime.
But it won’t tell you, on its own, what our Universe is made of or how our Universe is behaving. To know that, we have to inform ourselves by looking at the Universe we have, and determining what’s in it and where.
For example, we live in a Universe that has roughly the same amount of matter, on large scales, in all directions and at all locations in space. A Universe that has those properties — that’s the same in all locations (homogeneous) and in all directions (isotropic) — cannot be static and unchanging. Either the spacetime itself will contract, leading to a collapsed object of some type, or it will expand, with objects appearing to recede from us faster and faster the farther away from us they are.
The only way we know this to be true, however, is from our observations. If we didn’t observe the Universe and notice that the farther away a galaxy is from us, on average, the greater the amount that its light is redshifted, we wouldn’t have concluded that the Universe is expanding. If we didn’t see, on the largest scales, that the Universe’s average density was uniform to a 99.99%+ precision, we wouldn’t have concluded that it’s isotropic and homogeneous.
And in the places where, locally, enough matter has gathered in one place to form a bound, collapsed structure, we wouldn’t have concluded that there’s a supermassive singularity at the center if we didn’t have overwhelming observational evidence for supermassive black holes.
You might think of the famous image from the Event Horizon Telescope of this 6.5 billion solar mass behemoth at the center of Messier 87 when talking about supermassive black holes, but that’s just the tip of the metaphorical iceberg. Practically every galaxy out there has a supermassive black hole at their center. Our Milky Way has one that comes in at about 4 million solar masses, and we’ve observed it:
- indirectly, from stars moving around a large mass that emits no light at the galactic center,
- indirectly, from matter that falls into it and causes X-ray and radio emissions, including flares,
- and directly, with the same technology and equipment that measured the black hole at the center of Messier 87.
Many of us are hopeful that the Event Horizon Telescope collaboration will release an image of the Milky Way’s central black hole later this year. They have the data, but because it’s some ~1500 times less massive than the one we got our first image of, it changes on timescales that are ~1500 times faster. Producing an image that’s accurate will be a much greater challenge, especially given how faint this radio signal is in such a messy environment. Still, the team has expressed optimism that one will be forthcoming within the next few months.
The combination of direct and indirect evidence makes us more confident that the X-ray and radio emissions we are seeing from various sources throughout the Universe really are black holes. Black holes in binary systems emit telltale electromagnetic signals; we’ve discovered scores of them over the years. Active galactic nuclei and quasars are powered by supermassive black holes, and we’ve even observed them turning on and off as matter either begins or ceases to feed these central engines.
In fact, we’ve observed “radio-loud” supermassive black holes in a myriad of galaxies wherever we look. A new survey from the LOFAR array, for example, has begun surveying the northern celestial hemisphere, and with only a tiny fraction of the sky under their belt, they’ve already discovered more than 25,000 supermassive black holes. From a map of them, you can even see, already, how they clump and cluster together, following the large-scale distribution of massive galaxies in our Universe.
All of this discussion of black holes doesn’t even include the most revolutionary development of the past decade: the direct detections we’ve made using gravitational wave observatories. When two black holes inspiral and merge, they create gravitational waves: ripples in spacetime, a completely novel, non-electromagnetic (light-based) form of radiation. When those ripples pass through our gravitational wave detectors, they alternately expand and compress the space present in different directions, and we can see the patterns of those ripples in our gravitational wave data.
Right now, the only successful detectors we have are those under the guidance of the LIGO and Virgo collaborations, which are relatively small in scale. This limits the frequency of the waves they can observe, corresponding to low-mass black holes in the final stages of inspiral and merger. In the coming years, new, space-based detectors like LISA will take flight, enabling us to detect larger-mass black holes and to see them, and the smaller ones, long before the actual final moments of a merger occurs.
Meanwhile, there’s another enormous puzzle about our Universe: the dark matter problem. If we take into account all the matter that we know of and can directly detect — atoms, plasma, gas, stars, ions, neutrinos, radiation, black holes, etc. — it only accounts for about ~15% of the total amount of mass that must be there. Without about six times as much mass as we see, which cannot collide or interact the same way normal atoms do, we cannot explain:
- the fluctuation patterns seen in the cosmic microwave background,
- the large-scale clustering of galaxies and galaxy clusters,
- the motions of individual galaxies within galaxy clusters,
- the sizes and masses of galaxies that we observe,
- or the gravitational lensing effects of galaxies, quasars, or colliding galaxy groups and clusters.
Adding in just one new ingredient, some form of cold, collisionless dark matter, explains all of these puzzles in one fell swoop.
Yet, somehow, this is still dissatisfying in a sense. We know some general properties of what dark matter should be that, combined, all tell a compelling story about the Universe. But we have yet to directly detect whatever particle might be responsible for it. A species of matter that’s purely collisionless doesn’t necessarily explain the cosmic structure that appears on the smallest scales. It’s possible that there are purely gravitational effects — like dynamical heating — that are responsible for this mismatch, but it’s also more possible, and perhaps even more likely, that dark matter is not quite so simple.
Meanwhile, on the black hole side, we now see many supermassive black holes that are somehow grew to be a billion solar masses or more in just a few hundred million years: a tremendous puzzle for structure formation in our Universe. Based on our understanding of the first stars and how the earliest black holes would arise from them, we simply struggle to explain how they got to be so big so fast, as we see these behemoths at significantly earlier times than anticipated.
These are the frontiers of our knowledge, and represent some of the most pressing problems in modern cosmology today. We’ve come as far as we have because of the observatories, tools, and discoveries that have already occurred, and our knowledge of the laws of physics that helps us interpret them and place them in their proper context. On the other hand, there’s a lot to be excited about as far as new technological developments and observational capabilities on the very near-term horizon. This is a big deal; we’re at the frontiers of our everlasting quest to understand the Universe around us!
That’s why I’m excited to be live-blogging a talk on The Invisible Universe by PhD astronomer and Yale professor Priyamvada Natarajan. One of the top observational cosmologists today, she has a recent book out called Mapping the Heavens: The Radical Scientific Ideas that Reveal the Cosmos. Her talk, available to the public, occurs at 7 PM ET/4 PM PT on March 3, 2021, courtesy of Perimeter Institute.
Tune in then and follow along starting at 3:50 PT (all times to follow in Pacific Time) then, where I’ll be live-blogging the talk from a theoretical cosmologist’s perspective!