So your friend asks, "What's with dark matter and dark energy?"

by Sarah Scoles

So you're hanging out with your friends, and that one guy says, "Hey, did you know that 95% the universe is dark matter?"

You throw up your hands and say, "Hey, buddy, hang on just one second. That's not true. What about dark energy?"

He says, "What about what? Who cares? It's all dark."

To win the argument that will surely ensue, you have to have some evidence. Read on to find out what dark stuff, exactly, makes up 96% of the universe.

Dark Matter + Dark Energy

It is true that most of the universe is made up of things that can't be seen, and whose presence is inferred by its effects on the things that can be seen. But what effects do we see? And how does that translate to a percentage of the universe that remains a mystery?

Why do we think dark matter exists?

Galaxies rotate, and when we use gravitational laws to predict what that rotation should look like, we find that they should behave like the solar system--the farther away something is from the central mass (in the case of the solar system, the sun, and in the case of a galaxy, the supermassive black hole at the center), the slower it should orbit; the gravitational force on Pluto, for instance, is much smaller than the gravitational force on Mercury, because it's much farther away from the sun. A star at the fingertip of an arm of the galaxy should orbit more slowly than we do. However, that's not really what happens: galaxies have flat rotation curves, meaning that objects farther away from the supermassive black hole don't really orbit more slowly than things closer to it. For a more detailed treatment of this topic, check out my previous post about rotation curves. What this implies is that there is lots of mass spread throughout the galaxy--that most of the mass isn't just at the center--and that the spread-out mass has a large gravitational effect on the galaxy's rotation.

There is also evidence on larger scales--namely, galaxy clusters, or little neighborhoods of galaxies that are gravitationally bound together. 

The large-scale filamentary structure of the universe, which looks a little something like this, blows my mind every time, because it looks like the mind--neurons or galaxies? For Thanksgiving, I'm thankful that the universe appears dendritic. Source.

The way galaxy clusters behave--basically, how they cluster, and the way they are able to stay clustered--implies that there is more matter than the matter we can see (stars, gas, etc.). The way astronomers measure this discrepancy is called the "mass-to-light ratio." A certain amount of light, produced by the stars and the gas, implies a certain amount of mass. But the motions of galaxies imply a different amount of mass. It is the relationship between these two different quantities that tells us how much of the mass must be "dark."

The last important reason many scientists invoke the existence of dark matter is that galaxies and clusters formed at all. The cosmic microwave background (CMB), or the "glow" seen in every direction in the sky, is leftover evidence of the Big Bang. Small fluctuations in the CMB grew into the un  iversal structures that we see today, and that growth requires something besides the mass that we can see. For more about these fluctuations and their evolution, see this previous post .

In all cases, either our theories of gravitation are wrong, and you shouldn't sleep well at night, or there is missing mass, and you shouldn't sleep well at night.

Does dark matter relate at all to the universe's expansion?

Why, yes, it does. And so does dark energy. The universe's expansion is where these two dark things come together. The more matter there is, the more gravity holds the universe together. But there is not enough matter, even with the addition of this dark stuff, to keep the universe attracted enough to keep itself from expanding.

What's up with the universe's expansion? Didn't I hear something about a Nobel Prize for that?

I'm glad you read the news. It's true: this year, Saul Perlmutter, Brian P. Schmidt, Adam G. Riess won the Nobel Prize for their discovery that the universe is not only getting larger, but getting larger faster than it used to be getting larger. In other words, the universe's expansion is accelerating. Seriously, I don't know how anyone sleeps at night. 

How did they discovery something so nightmarishly crazy and amazing?

The Hubble Space Telescope took observations of far-away supernovae, Type Ia. In this type of supernova, a white dwarf and a regular star are in a binary orbit. The normal star deposits material onto the white dwarf star until the white dwarf star attains ~1.4 solar masses, and then BOOM THERMONUCLEAR EXPLOSION. 

This 1.4 solar masses is called the Chandrasekhar limit, and since the EXPLOSION always happens right around this limit, all Type Ia supernovae put out the same amount of light. Their bright sameness makes them a standard candle--since we know how bright they would be if we were right next to them, we can figure out (by how bright they appear when their light gets to us) how far away they are. It's like if you knew a lightbulb in your living room was putting out 60W, and you stepped into kitchen and measured how many Watts reached you, you could figure out how far away the kitchen was (obviously a very important distance to know precisely).

Matters of life and death. Source.

If we know how far away the supernovae are, we can find out when they actually EXPLODED. And by measuring the light, we can also see how far redshifted it is, which tells us how fast the system is moving away from us. And, in this cosmological case, how much expanding the universe has done since the explosion.

But, like with mass and gravity, the results lead to confusion. If the universe's expansion were slowing, the redshift would imply that the supernova should be brighter than observed. The redshift would, in other words, imply that the supernova was farther away than it actually was (meaning that the system had not been carried by the expansion of the universe as far as it would have been if the expansion were constant, or increasing). However, what the astronomers actually saw was that the supernovae were dimmer than the redshift indicated, meaning that the system had been carried farther than it would have been if the expansion were constant or decreasing.

What is causing the expansion to accelerate?

If gravity were the only thing hanging out in the large-scale and being powerful, the mass in the universe would be attracted enough to itself (a common problem) that the universe would slow its roll or collapse in on itself. That's what scientists expected. Since that's not what they saw, the theory needed to be adjusted to include a force that could overrule the overlord of gravity.

Dark energy is repulsive: while gravity wants to pull together, dark energy wants to push apart. Dark energy is winning, which is why the universe is on a possibly neverending downhill roller coaster.

After all, if dark energy is a fundamental property of space itself, which it appears to be, the more space that exists, the more dark energy there is. And the more dark energy there is, the bigger space gets. And, consequently, the less dense with matter the universe is, as all the matter is getting farther apart. This, friends, is the definition of a vicious cycle.

So what are the stats?

Dark energy accounts for about 73% of the total mass-energy density of the universe, while dark matter accounts for about 23%.

Dark matter accounts for about 83% of the matter in the universe.

Below are some papers about these topics, if you'd like to dive deeper into the rabbit hole. Do so at your own risk.

Riess, A., Filippenko, A., Challis, P., Clocchiatti, A., Diercks, A., Garnavich, P., Gilliland, R., Hogan, C., Jha, S., Kirshner, R., Leibundgut, B., Phillips, M., Reiss, D., Schmidt, B., Schommer, R., Smith, R., Spyromilio, J., Stubbs, C., Suntzeff, N., & Tonry, J. (1998). Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant The Astronomical Journal, 116 (3), 1009-1038 DOI: 10.1086/300499

Perlmutter, S., Aldering, G., Goldhaber, G., Knop, R., Nugent, P., Castro, P., Deustua, S., Fabbro, S., Goobar, A., Groom, D., Hook, I., Kim, A., Kim, M., Lee, J., Nunes, N., Pain, R., Pennypacker, C., Quimby, R., Lidman, C., Ellis, R., Irwin, M., McMahon, R., Ruiz‐Lapuente, P., Walton, N., Schaefer, B., Boyle, B., Filippenko, A., Matheson, T., Fruchter, A., Panagia, N., Newberg, H., Couch, W., & Project, T. (1999). Measurements of Ω and Λ from 42 High‐Redshift Supernovae The Astrophysical Journal, 517 (2), 565-586 DOI: 10.1086/307221

Rubin, V., & Ford, W. (1970). Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions The Astrophysical Journal, 159 DOI: 10.1086/150317

Comments

[Abe]

"Filippenko, A." referenced in the articles is UC Berkeley Professor Alex Filippenko, a Simpsons fan, a great uncle of mine through two marriages and several degrees of cousins, and an all around awesome guy. His is by far the most popular class at Berkeley because he dedicates the same degree of scientific analysis and testing to his research as to his teaching. GREAT teacher, incredible researcher, good stuff.

Thanks for the article, came here through Reddit.com

[Sarah Scoles]

Great to hear that Filipenko is a great person as well as a great researcher and teacher! Thanks for letting us know.

[Anonymous]

So glad that I spent the time to read this. :)

[Anonymous]

joe shane

[D.J. Pisano]

Great post and description of the evidence for dark matter and dark energy! I just wanted to add some detail on further evidence for dark matter. While the motions of galaxies in galaxy clusters is most easily explained by the presence of dark matter, most of the normal, baryonic matter in clusters is in the form of hot, x-ray emitting gas. If MOND (mentioned in your previous post) is correct and this x-ray gas provides the mass in a cluster, some dark matter would still be needed to explain the motions of the galaxies.

There is another nice example for why dark matter must exist in galaxy clusters. The bullet cluster is a case where two clusters have crashed into each other. The x-ray gas in the two clusters slammed together and stayed in the middle, while the galaxies, with their associated dark matter (as traced by gravitational lensing of the light behind the cluster) pass right through each other. This result is difficult to explain by MOND.

Despite all of this evidence there are still lots of challenges for current theories of dark matter. For example, there are far too many small galaxies predicted to exist as compared to what has been observed. And systems such as the bullet cluster are predicted to be extremely rare.

Finally, since energy is equivalent to matter according to Einstein's famous E=mc^2 equation, the presence of dark energy and dark matter help provide enough "mass" to cause the universe to have the shape most theorists would like it to have.

Keep up the good work!

[Anonymous]

Loved this article, or whatever else it may be called. I'm quite curious as to where those stats came from though, if anyone else knows.