Chapter 19: Our Expanding Universe

One of the triumphs of the 20th century was the development of a scientific theory for the origins and evolution of the universe. The Big Bang theory has made many predictions, all of which have been verified by astronomical observations. The Cosmic Background Radiation (CBR shown above) was predicted in 1948 but wasn’t observed until 1965.

The Cosmological Principle is our starting point

The cosmological principle is that the universe is homogeneous (the same everywhere) and isotropic (the same in all directions).

It is possible to be homogeneous but not

isotropicThe Cosmological Principle says the universe is both homogeneous and isotropic. It is possible to imagine a situation which is homogeneous but not isotropic. The bricks are the same everywhere so it is homogeneous. There are preferred directions, though, so it isn’t isotropic.

It is also possible to be isotropic but not homogeneous

In this situation we have an isotropic universe if we are at the center. If I am at the center, it looks the same in all directions so it is isotropic. It is definitely not the same everywhere, though, so it isn’t homogeneous

Is the universe static?For much of human history we believed the universe was static and never changing. This is called superhomogeneous: the universe not just the same everywhere and in all directions but also everywhen The ancient celestial sphere model is a static universe.

Isaac Newton envisioned a static universe in perfect

gravitational balanceIsaac Newton thought the universe was an infinite expanse of stars uniformly distributed throughout space. His theory of universal gravitation required the stars to be uniformly distributed or their mutual gravitation would cause them to collapse to a central point.

Einstein added a “fudge factor” to make his equations of general relativity give a static universe

Einstein’s theory of gravity, general relativity, predicted that the universe could not be static just as Newton’s gravity did. It was possible to make the universe static by adding a constant to his equations so he did it. He called it the cosmological constant.

The Shapley – Curtis Debate

By the 1920’s astronomers had built large enough telescopes that they could see details in objects they called spiral nebulae. They had only recently (in the previous decade) learned the true size of the Milky Way so the question was “Are the spiral nebulae inside the Milky Way or outside it?” A great debate on the matter was held in Washington DC in April 1920 and astronomers from all over the country came.

In the late 1920’s Edwin Hubble, working with Vesto

Slipher, discovered the expansion of the universe

Hubble measured the distance to nearby galaxies and Slipher measured their spectra

Edwin Hubble measured Cepheid variable stars in galaxies to get

their distance from us

brightness4

LuminosityDistance

Cepheids are supergiant stars so they are visible at tremendous distances. Hubble was able to resolve individual Cepheids in several nearby galaxies and thereby determine the distance to those galaxies.

Vesto Slipher’s galaxy spectra showed that the

galaxies seem to be moving away from us

At the same time that Hubble was measuring the distance to nearby galaxies, Vesto Slipher was measuring the spectra of those same galaxies

Hubble plotted the recessional velocity from Slipher’s spectra

against his distance measurements

Hubble’s genius was to combine his distance measurements with Slipher’s redshift measurements. When he plotted them against each other he got a straight line. The farther a galaxy was from us the faster it was moving away from us. From this he deduced that the universe must be expanding.

The slope of the galaxy velocity versus distance graph is now called the Hubble Constant

0v H dIn honor of Edwin Hubble, we call the constant which comes from the slope of the velocity versus distance graph the Hubble Constant.

What this means is that the universe is expanding

An analogy that is often used to illustrate the expanding universe is galaxies drawn on the surface of a balloon. As the balloon is inflated, the galaxies all move away from each other.

How do we measure the distance to galaxies?

Cepheid variable stars are very bright and can be seen from a long distance. The image above shows how the HST was able to resolve a Cepheid variable in M100, a galaxy 60 million lightyears away. This is close to the limit for the HST, though. 60 Mly may seem like a very large distance but in cosmology, that is considered “nearby”. So how do we measure the distance to a galaxy that may be over a billion lightyears away?

The brightest things we have seen so far are supernova. Can

we use them as “standard candles”?

There is too much variability in Type II supernovae to use them as standard candles. They can be stars with 10 solar masses or stars with 100 solar masses.

Type I supernovae make good standard candles because they are always the same mass when they explode: 1.4 Msun.

We have developed a distance ladder that uses different techniques to determine

distance

If we run the expansion movie backwards, everything gets closer

We can use the Hubble constant to get an estimate of the age of the universe. Since the universe is expanding, if we go backwards in time it will be contracting. If we go far enough back, the entire universe we see now must have been the size of a single point

The Big Bang was not an explosion at a point in space

The Big Bang occurred at all points in the universe. It is the expansion of space-time, not the movement of galaxies. Most of us can picture an explosion but the Big Bang was not a conventional explosion. Think of it this way: most astronomers believe the universe is infinite. If it is infinite now, it was infinite at the moment of the Big Bang. Everything we see in the universe came from a single point at that instant but there were an infinite number of points in the infinite universe at the moment of the Big Bang.

It is space-time that is expanding

Here is another way of trying to understand the expansion of the universe. Recall that Einstein had two important theories: special relativity which deals with things moving at speeds close to the speed of light and general relativity which is his theory of gravity. The expanding universe is a General Relativity effect, not a special relativity effect

We measure the expansion through a scale factor

The scale factor is like the ruler we use to measure the distance between thing in the universe with.

Galaxy redshifts allow us to measure the scale factor

The ruler we can use to measure the scale factor with is the redshift of light.

If the universe began in a Big Bang, what kind of

predictions does the theory make?

The first prediction was by George Gamow and Ralph

AlpherIf the visible universe was very small at some time then it must have been very hot. If it was hot, it would have glowed everywhere by its blackbody spectrum. Gamow and Alpher predicted that the glow should come from a time when the entire visible universe was a few thousand degrees making it glow mostly in the visible wavelengths. In the billions of years since then the wavelength of the glow should have been stretched by the expansion of space into the microwave region of the spectrum. They made the prediction in 1948.

The Alpher, Bethe, Gamow paper predicted a cosmic

background radiation

Alpher and Gamow added Hans Bethe to the list of authors on their paper as a joke. In the paper they predicted that the sky should be filled with faint microwaves they called the Cosmic Background Radiation (CBR).

Arno Penzais and Robert Wilson were the first to observe the

CBR

Arno Penzais and Robert Wilson were scientist working for Bell Labs trying to bounce microwaves off sounding balloons in the early 1960’s. They were working in the microwave wavelength range but were not looking for the CBR. In fact, they hadn’t even heard of the CBR.

The CBR comes from the Era of Recombination

Recombination is actually a misnomer. Before this time the universe was too hot for electrons and protons to

combine together into atoms. After the universe had expanded and cooled for about 300,000 years, the temperature fell enough that the hydrogen atoms could survive.

Before Recombination it was like being in a fog

In a fog, light (photons) can’t travel very far before being scattered off tiny water droplets. In a similar manner, before Recombination, photons couldn’t travel very far before being scattered by an atom (and thus breaking the atom up).

In the 1989 The COBE

satellite was launched to observe the CBR in detailEarly observations of the CBR

showed it to be extremely uniform. The Big Bang theory predicted it should have some very small variation, though. The Cosmic Background Explorer (COBE) was launched in 1989 to measure those small variations

The COBE found the CBR to have a perfect blackbody

spectrumWhen the COBE data came in, astronomers were amazed at how perfectly it fit the spectrum of a blackbody at 2.74 Kelvin.

When looked at very closely, there are variations in the

CBR

The variation is only a few parts per million but it is there, just as predicted. The COBE satellite didn’t have the best resolution, though, so an even more sensitive microwave telescope was sent up with even higher resolution.

The WMAP mission got an even higher resolution look at the CBR

Observations of the CBR continue to improve with the

Planck Telescope

The Planck has slightly higher resolution and sensitivity than the WMAP had

The second prediction of the Big Bang theory is about the

contents of the universeWhy are hydrogen and helium the most common elements in the universe? When we looked at the lives and deaths of stars we saw that all the elements beyond helium come from fusion in the cores of stars or their supernova deaths. But where did the hydrogen and helium come from?

In the early universe it was hot enough for hydrogen

fusionIn the first few millionths of a second, subatomic particles called quarks were created and they combined to form protons and neutrons. As the universe expanded and cooled, the temperature fell to around 20,000,000 K and the protons (hydrogen nuclei) could fuse into helium.

The predicted quantities match the observed values

Measuring the relative abundance of various isotopes in the universe is not an easy task but some researchers have attempted it. The boxes represent the observed numbers while the blue bar is the best fit to all the observations. The colored lines are the predictions of the Big Bang theory.