Dark matter, dark energy, and the fate of the Universepeople.physics.tamu.edu/quadri/astr101_fall16/files/18_cosmology.pdfWe call that moment the big bang. Review: early stages of

Dark matter, dark energy, and the fate of the Universe

Q: How can you find the age of the Universe?A. Take the distance to a nearby galaxy, divide it by its

speed of recession (time = distance/velocity).B. Take the distance to a far-away galaxy, divide it by

its speed of recession (time = distance/velocity).C. Take the distance to any galaxy, divide it by its

speed of recession (time = distance/velocity).

Q: How can you find the age of the Universe?A. Take the distance to a nearby galaxy, divide it by its

speed of recession (time = distance/velocity).B. Take the distance to a far-away galaxy, divide it by

its speed of recession (time = distance/velocity).C. Take the distance to any galaxy, divide it by its

speed of recession (time = distance/velocity).

Q: What are the slight fluctuations seen in maps of the cosmic background radiation?

A. Uncertainties in the mapB. Variations in the instrument's sensitivityC. The beginning of the formation of galaxies and

clusters of galaxiesD. Dark matterE. None of the above

Q: What are the slight fluctuations seen in maps of the cosmic background radiation?

A. Uncertainties in the mapB. Variations in the instrument's sensitivityC. The beginning of the formation of galaxies and

clusters of galaxiesD. Dark matterE. None of the above

Review: the expansion of the universe

By measuring the distances to other galaxies, Edwin Hubble found that their recessional velocity is proportional to their distance

Hubble’s law:v=Ho×dwhere Ho is Hubble’s constantHo=22/km/s/Mly

Review: the big bangRight now we see everything flying apart. But if you were to run the clock backwards, then everything would be flying toward one another, until everything in the observable universe is compacted into a single point. We call that moment the big bang.

Review: early stages of the Universe

• T<=5 minutes: nuclear fusion could take place. At the end of this period, the Universe consisted of 75% hydrogen and 25% helium.

• T=380,000 years: up until this time photons could not stream freely through the Universe. But then the electrons become bound to nuclei, and the Universe becomes transparent. This is when the cosmic microwave background radiation comes from.

Review: the light left over from (shortly after) the big bang

The case was closed in favor of the big bang theory with the discovery of the cosmic microwave background (CMB) radiation in 1964.

hot spots

cold spots

The history of the Universe

Review: Olber’s paradoxIf the Universe if infinite, then in every single direction that you can look all you should be able to see is stars.

The fact that we do not see an infinite number of stars is due to the fact that we can not see infinitely far — we can only see back to the big bang! (or technically, to the CMB)

The expanding Universe

• The standard assumption had always been that the Universe is eternal and unchanging. The Universe had no beginning, and has no “fate.”

• Einstein knew that his theory of gravity showed that the Universe had to be either expanding or contracting — and that it could not be static. But he couldn’t believe it.

• So he just added in an extra term into his equations, called the cosmological constant, that acts as a repulsive force. This force would counteract gravity, allowing the Universe to remain static.

The expanding UniverseWith the discovery that the Universe is expanding, Einstein realized that his original equation was right; he should have known that the Universe isn’t static. He called this his “biggest blunder.”

D’oh!

The fate of the Universe

A major driver for astronomical research in the 20th century was to figure out the fate of the Universe. There were basically three options:

1. The Universe will continue to expand forever.2. The Universe will continue expanding but gravity

is slowing down the expansion; eventually (as time→∞) it will come to a halt. (We call this a critical universe.)

3. Gravity will reverse the expansion, eventually causing a big crunch.


A major driver for astronomical research in the 20th century was to figure out the fate of the Universe. There were basically three options:

1. The Universe will continue to expand forever.2. The Universe will continue expanding but gravity

is slowing down the expansion; eventually (as time→∞) it will come to a halt. (We call this a critical universe.)

3. Gravity will reverse the expansion, eventually causing a big crunch. And maybe, this will be followed by another big bang.


Q: What is the fate of the Universe?A. The Universe will continue to expand forever.B. The Universe will continue expanding but gravity

is slowing down the expansion; eventually (as time→∞) it will come to a halt.

C. Gravity will reverse the expansion, eventually causing a big crunch… perhaps followed by another big bang.

D. I already know what the answer is.


So how do we figure out which of these three cases is correct? It all came down to two things: how fast the Universe is expanding, and how dense it is.1. If the Universe is expanding rapidly, and if the density

is low enough, then gravity can’t halt the expansion.2. If the expansion is a bit slower, or if the Universe is a

bit more dense, then gravity will eventually bring the expansion to a halt (but only at time=∞).

3. If the expansion is slow enough, or if the Universe is dense enough, then gravity will reverse the expansion.


So a huge amount of effort has gone into measuring the expansion rate Ho. Getting a good measurement is actually very difficult, but it seemed to be a pretty big number.


So a huge amount of effort has gone into measuring the expansion rate Ho. Getting a good measurement is actually very difficult, but it seems to be a pretty big number…

We also estimated the mass density of the Universe by adding up all the luminosity from the galaxies in some volume. This seemed to be a pretty small number…

Fast expansion + not much gravity to slow it down ➔ the Universe will expand forever!

But wait!

A new, mysterious form of matter

In the 1930s Fritz Zwicky was studying a cluster of galaxies, and found that the galaxies were moving around much faster than expected. So fast that the whole cluster should have flown apart.

(1898-1974)


He figured there must be a very strong gravitational force holding it together, and therefore that there must be some other form of mass in the cluster that that didn’t emit light. He called it dark matter.

(1898-1974)


Zwicky’s conclusion remained a curiosity until the 1970s, when Vera Rubin found a similar effect when studying the velocities of stars orbiting around the centers of galaxies. They were moving much too fast!

(1028-current)

what we actually observe

distance from center of galaxy

orbi

tal v

eloc

ity

what we expect based on the amount of mass we see


We eventually concluded that there is roughly 5x more mass in the Universe than we can see. We call this dark matter, simply because it doesn’t emit light. So what is the dark matter?

• It could be large objects, like planets, brown dwarfs, or black holes

• Or it could be tiny objects: a new type of particle


We eventually concluded that there is roughly 5x more mass in the Universe than we can see. We call this dark matter, simply because it doesn’t emit light. So what is the dark matter?

• It could be large objects, like planets, brown dwarfs, or black holes

• Or it could be tiny objects: a new type of particle

After extensive searches for larger objects, we haven’t found them. We conclude that the dark matter must be particles that we haven’t been able to detect yet.

The fate of the Universe.

But the 1990s we had made pretty good measurements of H0, and had a decent idea of the total amount of matter (normal+dark) in the Universe. So what is the answer? Is there enough dark matter in the Universe to halt the expansion?

Nope, not even close. Even with all the dark matter, we still need about 4x more mass to halt the expansion. The Universe will expand forever.

But wait!

A new, mysterious form of energyIt seemed like there wasn’t enough mass in the Universe to halt the expansion. But it should be enough to slow the expansion down. So in the 1990s astronomers began to try to measure what the expansion looked like in the past.

A new, mysterious form of energyIt seemed like there wasn’t enough mass in the Universe to halt the expansion. But it should be enough to slow the expansion down. So in the 1990s astronomers began to try to measure what the expansion looked like in the past.

A new, mysterious form of energy

• We now think that there is some completely different kind of energy that permeates all of space. This energy has the effect of antigravity; it pushes things apart.

• There is about 3x more dark energy than there is in normal and dark matter combined.

So maybe Einstein’s “biggest blunder” wasn’t really such a blunder after all?

A new, mysterious form of energy

D’oh!D’oh!

The unknown UniverseThis leaves us in a pretty embarrassing situation:

• All of the matter that we see and know about is only about 1/6 of all the matter that there is.

• And all the matter that there is is only about 1/4 of all the stuff there is.

Most of the Universe is unknown to us.


We now know that the Universe will expand forever, and in fact the expansion is going to get much faster. The Universe is going to rip itself apart…


We now know that the Universe will expand forever, and in fact the expansion is going to get much faster. The Universe is going to rip itself apart…

In the next few billion years, the Milky Way and Andromeda will merge to form a single large elliptical galaxy (“Milkomeda”).

The fate of the Universe: the next few 10s of billions of years

• Andromeda_and_Milky_Way_collision.ogg


Since most of the gas in galaxies has turned into stars, there is not much gas left over. So the star formation rate in galaxies is decreasing.



When current massive stars expel some of their material in a supernova or a planetary nebula, that material will be recycled into a small number of new stars. But current low-mass (and very red) stars will continue to burn for 100s of billions of years.

Eventually there will only be these low-mass red stars left.

• Because of dark energy, galaxies that aren’t gravitationally bound to each other are going to start moving apart extremely rapidly.

• Soon they’ll be flying away from us at faster than the speed of light… which means any observers left in Milkomeda wouldn’t be able to seem them! These observers might think that Milkomeda is the only galaxy in the Universe…


The fate of the Universe: the next few trillions of years

Eventually all stars will die, leaving a bunch of cold planets, brown dwarfs, white dwarfs, neutron stars, and black holes. (And there will be some gas left over but it will be too diffuse to form new stars.)

• Eventually all particles (that make up planets, neutron stars, etc.) will decay into photons. These photons will get severely redshifted.

• All that will be left is black holes. But even those will decay into photons.

• The ultimate fate of the Universe is that it will be filled with very long-wavelength photons and subatomic particles, which will be separate by great distances. Nothing will ever happen, and there will be no past and no future.

The fate of the Universe: the next 10100 years

Documents

Dark matter, dark energy, and the fate of the Universepeople.physics.tamu.edu/quadri/astr101_fall16/files/18_cosmology.pdfWe call that moment the big bang. Review: early stages of