What Astronomers Wish Everyone Knew About Dark Matter And Dark Energy
Among the general public, people compare it to the aether, phlogiston, or epicycles. Yet almost all astronomers are certain: dark matter and dark energy exist. Here’s why.
If you go by what’s often reported in the news, you’d be under the impression that dark matter and dark energy are houses of cards just waiting to be blown down. Theorists are constantly exploring other options; individual galaxies and their satellites arguably favor some modification of gravity to dark matter; there are big controversies over just how fast the Universe is expanding, and the conclusions we’ve drawn from supernova data may need to be altered. Given that we’ve made mistaken assumptions in the past by presuming that the unseen Universe contained substances that simply weren’t there, from the aether to phlogiston, isn’t it a greater leap-of-faith to assume that 95% of the Universe is some invisible, unseen form of energy than it is to assume there’s just a flaw in the law of gravity?
The answer is a resounding, absolute no, according to almost all astronomers, astrophysicists, and cosmologists who study the Universe. Here’s why.
Cosmology is the science of what the Universe is, how it came to be this way, what its fate is, and what it’s made up of. Originally, these questions were in the realms of poets, philosophers and theologians, but the 20th century brought these questions firmly into the realm of science. When Einstein put forth his theory of General Relativity, one of the first things that was realized is if you fill the space that makes up the Universe with any form of matter or energy, it immediately becomes unstable. If space contains matter and energy, it can expand or contract, but all static solutions are unstable. Once we measured the Hubble expansion of the Universe and discovered the leftover glow from the Big Bang in the form of the Cosmic Microwave Background, cosmology became a quest to measure two numbers: the expansion rate itself and how that rate changed over time. Measure those, and General Relativity tells you everything you could want to know about the Universe.
These two numbers, known as H_0 and q_0, are called the Hubble parameter and the deceleration parameter, respectively. If you take a Universe that’s filled with stuff, and start it off expanding at a particular rate, you’d fully expect it to have those two major physical phenomena — gravitational attraction and the initial expansion — fight against each other. Depending on how it all turned out, the Universe ought to follow one of three paths:
- The Universe expands fast enough that even with all the matter and energy in the Universe, it can slow the expansion down but never reverse it. In this case, the Universe expands forever.
- The Universe begins expanding quickly, but there’s too much matter and energy. The expansion slows, comes to a halt, reverses, and the Universe eventually recollapses.
- Or, perhaps, the Universe — like the third bowl of porridge in Goldilocks — is just right. Perhaps the expansion rate and the amount of stuff in the Universe are perfectly balanced, with the expansion rate asymptoting to zero.
That last case can only occur if the energy density of the Universe equals some perfectly balanced value: the critical density.
This is actually a beautiful setup, because the equations you derive from General Relativity are completely deterministic here. Measure how the Universe is expanding today and how it was expanding in the past, and you know exactly what the Universe must be made out of. You can derive how old the Universe has to be, how much matter and radiation (and curvature, and any other stuff) has to be in it, and all sorts of other interesting information. If we could know those two numbers exactly, H_0 and q_0, we would immediately know both the Universe’s age and also what the Universe is made out of.
Now, we had some preconceptions when we started down this path. For aesthetic or mathematically prejudicial reasons, some people preferred the recollapsing Universe, while others preferred the critical Universe and still others preferred the open one. In reality, all you can do, if you want to understand the Universe, is examine it and ask it what it’s made of. Our laws of physics tell us what rules the Universe plays by; the rest is determined by measurement. For a long time, measurements of the Hubble constant were highly uncertain, but one thing became clear: if the Universe were made 100% of normal matter, the Universe turned out to be very young.
If the expansion rate, H_0, was fast, like 100 km/s/Mpc, the Universe would only be 6.5 billion years old. Given that the ages of stars in globular clusters — admittedly, some of the oldest stars in the Universe — were at least 12 billion years old (and many cited numbers closer to 14–16 billion), the Universe couldn’t be this young. While some measurements of H_0 were significantly lower, like 55 km/s/Mpc, that still gave a Universe that was 11-and-change billion: still younger than the stars we found within it. Moreover, as more and more measurements came in during the 1970s, 1980s and beyond, it became clear that an abnormally low Hubble constant in the 40s or 50s, simply didn’t line up with the data.
At the same time, we were beginning to measure to good precision how abundant the light elements in the Universe were. Big Bang Nucleosynthesis is the science of how much relative hydrogen, helium-4, helium-3, deuterium, and lithium-7 ought to be left over from the Big Bang. The only parameter that isn’t derivable from physical constants in these calculation is the baryon-to-photon ratio, which tells you the density of normal matter in the Universe. (This is relative to the number density of photons, but that is easily measurable from the Cosmic Microwave Background.) While there was some uncertainty at the time, it became clear very quickly that 100% of the matter couldn’t be “normal,” but only about 10% at most. There is no way the laws of physics could be correct and give you a Universe with 100% normal matter.
By the early 1990s, this began to line up with a slew of observations that all pointed to pieces of this cosmic puzzle:
- The oldest stars had to be at least 13 billion years old,
- If the Universe were made of 100% matter, the value of H_0 could be no bigger than 50 km/s/Mpc to get a Universe that old,
- Galaxies and clusters of galaxies showed strong evidence that there was lots of dark matter,
- X-ray observations from clusters showed that only 10–20% of the matter could be normal matter,
- The large-scale structure of the Universe (correlations between galaxies on hundreds-of-millions of light year scales) showed you need more mass than normal matter could provide,
- but the deep source counts, which depend on the Universe’s volume and how that changes over time, showed that 100% matter was far too much,
- Gravitational lensing was starting to “weigh” these galaxy clusters, and found that only about 30% of the critical density was total matter,
- and Big Bang Nucleosynthesis really seemed to favor a Universe where just ~1/6th of the matter density was normal matter.
So what was the solution?
Most astronomers had accepted dark matter by this time, but even a Universe that was made exclusively of dark and normal matter would still be problematic. It simply wasn’t old enough for the stars in it! Two pieces of evidence in the late 1990s that came together gave us the way forward. One was the cosmic microwave background, which showed us that the Universe was spatially flat, and therefore the total amount of stuff in there added up to 100%. Yet it couldn’t all be matter, even a mix of normal and dark matter! The other piece of evidence was supernova data, which showed that there was a component in the Universe causing it to accelerate: this must be dark energy. Looking at the multiple lines of evidence even today, they all point to that exact picture.
So either you have all these independent lines of evidence, all pointing towards the same picture: General Relativity is our theory of gravity, and our Universe is 13.8 billion years old, with ~70% dark energy, ~30% total matter, where about 5% is normal matter and 25% is dark matter. There are photons and neutrinos which were important in the past, but they’re just a small fraction-of-a-percent by today. As even greater evidence has come in — small-scale fluctuations in the cosmic microwave background, the baryon oscillations in the large-scale structure of the Universe, high-redshift quasars and gamma-ray bursts — this picture remains unchanged. Everything we observe on all scales points to it.
It wasn’t always apparent that this would be the solution, but this one solution works for literally all the observations. When someone puts forth the hypothesis that “dark matter and/or dark energy doesn’t exist,” the onus is on them to answer the implicit question, “okay, then what replaces General Relativity as your theory of gravity to explain the entire Universe?” As gravitational wave astronomy has further confirmed Einstein’s greatest theory even more spectacularly, even many of the fringe alternatives to General Relativity have fallen away. The way it stands now, there are no theories that exist that successfully do away with dark matter and dark energy and still explain everything that we see. Until there are, there are no real alternatives to the modern picture that deserve to be taken seriously.
It might not feel right to you, in your gut, that 95% of the Universe would be dark. It might not seem like it’s a reasonable possibility when all you’d need to do, in principle, is to replace your underlying laws with new ones. But until those laws are found, and it hasn’t even been shown that they could mathematically exist, you absolutely have to go with the description of the Universe that all the evidence points to. Anything else is simply an unscientific conclusion.
Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.