The Dark Universe

FAS Astronomers Blog, Volume 31, Number 3.

This is part 2 of a 5-part series on the Universe (The Visible Universe, The Dark Universe, The Microscopic Universe, The Expanding Universe, and The Multiverse).

There is much we can see in the visible universe. On the other hand, the universe is also dark and mysterious. And it is so in several different ways.

If we look up at the sky at night, we notice it is dark. Of course, this is because the Sun is shining on the other side of the Earth. Wait a few hours and the Sun rises (well the Earth turns) and everything is bathed in light.

When we look out into the universe, we see objects because of the light they emit or the light that is reflected toward us. However, light is not always visible to our naked eyes. Light is a form of electromagnetic radiation that ranges from long wavelengths (radio waves and microwaves) all the way to short wavelengths (ultraviolet, x-rays, and gamma rays). Visible light is just a small sliver in between. All forms of electromagnetic radiation “rain down” on the Earth – not just the visible light we can see. Although much of it, other than visible light along with some radio and microwaves, is blocked by the Earth’s atmosphere. I guess we can say that much of it is dark, at least for what we can see with our eyes.

Yet there is much more to what we think of as the dark universe.

Everything we see in the universe is called baryonic matter – it is made up of mostly baryons. These are particles such as protons and neutrons, which are in turn made up of three quarks. A proton is two up quarks and a down quark. A neutron is two down quarks and an up quark. Protons and neutrons form atomic nuclei, which combine with electrons to form atoms. They in turn form molecules, and so on. You can learn more quarks and other fundamental particles in The Standard Model of Particle Physics.

Here’s where it gets interesting. Baryonic matter makes up only around 4% to 5% of the universe. Astronomers think the rest of the universe is dark. It is dark in the sense that we can’t see it, but it is called dark because we really don’t understand what it is.

A lot of this has to do with gravity. Gravity is the weakest of the four fundamental forces (Strong, Weak, Electromagnetic, and Gravity). When we look at all the baryonic matter (again the stuff we can see), it doesn’t generate enough gravity to explain how things work. The universe is also behaving in a way counter to what gravity should be doing.

Dark Matter

Gravity holds galaxies and galactic clusters together. However, there is a problem. Visible matter doesn’t appear to have enough gravity to do the job.

• In 1933, Fritz Zwicky determined that the Coma galactic cluster shouldn’t be there – the galaxies within it should be flying apart.
• In 1970, Vera Rubin and Kent Ford noticed that the stars in the Andromeda galaxy move at the same rate regardless of their distance from the center. This violates Kepler’s law of planetary motion that governs the way planets orbit around stars.

So, astronomers started to hunt for an explanation. They concluded that there is something out there that they called Dark Matter. It appears to be the scaffolding upon which galaxies and galactic clusters are built. It doesn’t interact with light, but it does with gravity.

• It provides the “missing mass” and the necessary gravity to hold galaxies and galactic clusters together.
• It exists as a halo around galaxies, so stars will move at the same rate regardless of their distance from the center.
• It turns nearby galaxies into gravitational lenses by bending light from distant galaxies.

Astronomers have some theories as to what dark matter is.

• MACHOS (Massive Compact Halo Objects) – normal matter that doesn’t emit much light including brown dwarfs and black holes.
• Neutrinos – ghostly almost massless particles that are everywhere. This is the theory of hot dark matter (HDM).
• WIMPS (Weakly Interacting Massive Particles) – large, more exotic particles that interact with gravity, but not with light. This is the theory of cold dark matter (CDM).
• Axions – hypothetical particles that preserve a feature of the strong nuclear force.

MACHOS and neutrinos have been ruled out. WIMPS and Axions are still in the running. However, despite many experiments, astronomers have failed to find direct evidence of dark matter particles. So, the search continues.

You can find some more discussion about this in an early article on Dark Matter.

Dark Energy

While dark matter explains why there seems to be too much gravity, another aspect of the universe appears to defy gravity.

In the late 1990s, two teams of astronomers decided to measure the extent to which gravity was slowing down the expansion of the universe.

• The Supernova Cosmology Project led by Saul Perlmutter, and
• the High-z Supernova Team led by Brian Schmidt and Adam Riess.

Measuring the distance to objects out in the universe isn’t that easy. The good news is that there are a series of yardsticks (aka standard candles) that can be used. These yardsticks provide overlapping scales giving us distance measurements far out into the cosmos. All this is explained in the article Cosmic Distances, Stellar Brightness, and The Hubble Constant.

• Parallax uses trigonometry to determine the distance to nearby stars that appear to shift position as the Earth travels around the Sun.
• Cepheid variables are stars that vary in brightness and their period of variation is related to their intrinsic brightness.
• Type 1A supernovae are massive explosions that occur when a white dwarf star absorbs material from a companion star. When the white dwarf reaches a specific mass, it explodes with a known brightness.

The two teams measured the apparent brightness of several type 1A supernovae. Because the intrinsic brightness is known, they determined the distance to these supernovae. They also measured the red shift of the light coming from the supernovae (or from the galaxies that contained the supernovae), which in turn determined how fast they are moving away from us.

They expected to find that the rate of expansion was slowing down as gravity pulled in on the distant galaxies. However, the measurements surprised everyone. Both teams found the same results and came to the same conclusion. The expansion of the universe wasn’t slowing down, it was speeding up. The universe is accelerating!

Just like dark matter, no one has any idea of what is causing this – so they called it Dark Energy. It does appear that gravity might have been slowing down the expansion for a while, but around 8 to 9 billion years after the Big Bang, dark energy seems to have kicked in and the universe began to accelerate.

There is some speculation that dark energy can take on different forms represented by two different concepts.

• The Cosmological Constant (Lambda). Einstein first proposed this in an attempt to fit his equations of general relativity to a static universe. It is now used in the standard model of the universe as the constant to account for dark energy. In fact, the current theory of the universe is called the LCDM (Lambda Cold Dark Matter) model to account for both dark energy and dark matter.
• Quintessence. This is a different form of dark energy that assumes it is not a constant but varies with time.

You can learn some more in an early article on Dark Energy.

When astronomers add up all the stuff in the universe, they count matter and energy together (remember Einstein’s e = mc²). They find that the universe is around 68% dark energy, 27% dark matter, and only 5% ordinary (baryonic) matter. Therefore, we’re just a small fraction of what’s out there and the rest is, well, dark.

Black Holes

While we’re on the subject of the dark universe, let’s consider one more thing – Black Holes.

Black holes are objects that are so dense, and their gravity is so intense, that nothing, including light, can escape. The surface of a black hole is described by its “event horizon”, which is located at the Schwarzschild radius from the center of the black hole. Karl Schwarzschild was the first to use Einstein’s general theory of relativity to predict the existence of black holes. Einstein’s theory says that matter bends/curves light. Black holes are so massive that light literally bends back in on itself.

Black holes are truly strange objects. They are places where stuff disappears from the universe. Where does it go? We really don’t know. What does the inside of a black hole look like? We really don’t know. Do the laws of physics hold within a black hole? We really don’t know, although we think they might not hold. If fact, the center of a black hole might be a singularity where the density is infinite.

There are two types (sizes) of black holes.

• Stellar-mass black holes, which form after very large stars explode as a type 2 supernovae.
• Supermassive black holes found at the center of galaxies.

In theory, any object could become a black hole if you squeeze it together enough. Once the entire mass of an object shrinks to within its event horizon, it becomes a black hole. In reality, it takes an incredible amount of force to accomplish this (e.g., a supernova), and even large objects such as the Earth would have to be squeezed into an extremely small volume (less than one inch in diameter).

Black holes themselves are black, but they can be seen (sort of).

• Material falling into a black hole tends to swirl around the black hole in the form of an “accretion disk” that we can see.
• Early supermassive black holes had so much material falling into them that they couldn’t absorb it all. Friction within the accretion disk sent out energetic streams of light and particles that we see as distance quasars.
• Black holes do collide. In doing so, they disturb space/time itself. They send out gravitational waves that can now be detected by observatories such as LIGO.

In the last few years, the Event Horizon Telescope captured and published images of two black holes (or more correctly the shadow of the black holes on their accretion disks). For more on this, see the article Sagittarius A* and the Event Horizon Telescope.

If you happen to fall into a black hole (this is not recommended), you will experience something called spaghettification. The tidal effects (difference in the back hole’s gravity between your feet and head) will stretch you out like a piece of spaghetti. You will probably experience other unpleasant things as well, so, again, this is not recommended. When you are spaghettified depends on the size the black hole.

• Stellar-mass Black Hole: The event horizon is close to its center, so spaghettification will happen well outside of the black hole.
• Supermassive Black Hole: The event horizon is large, so you might not notice anything as you cross it. Don’t worry, as you are pulled down toward the center by the gravitational forces, you will eventually be spaghettified and torn apart.

Einstein’s general theory of relativity has some interesting predictions about time as it relates to gravity. The more gravity, the slower time moves. We don’t notice this, but it is important for GPS satellites that experience a little less gravity while orbiting the Earth than what we feel on the surface.

For a black hole, this effect is huge. So much so, that if we saw someone falling into a black hole, it would appear that time stops for them. They would appear frozen as they cross the event horizon. They, however, wouldn’t notice it, and time would continue to move along (although they would be spaghettified and probably wouldn’t notice what was going on).

I’ve heard speculation that the inside of a black hole connects to a white hole somewhere else in the universe. This, as far as I can tell, is still in the realm of science fiction and not science fact.

The nearest black hole to the Earth is around 1,500 light years away, so you don’t have to worry about accidentally bumping into one. However, NASA has put together a guide to black hole safety in case you do end up getting too close to one (still not recommended.)

You can discover more about black holes in an early article.