The Birth of the Universe. Hubble Expansion and the Big Bang. The fact that more distant galaxies are moving away from us more rapidly indicates that the universe is expanding. This implies that the universe was born in a huge explosion, or Big Bang.
The fact that more distant galaxies are moving away from us more rapidly indicates that the universe is expanding. This implies that the universe was born in a huge explosion, or Big Bang.
If the Big Bang theory is correct, then the universe was very different in the past. We can test this prediction with images of the faintest and most distant galaxies, and hence looking back in time to when the universe was much younger.
To detect very distant galaxies, Hubble stared at small patch of sky for an entire week. This image, known as the Hubble Deep Field, shows that galaxies were smaller and more irregular in the past than they are today.
Another prediction of the Big Bang theory is that the universe was very hot immediately after it was born. As a result, the universe would have glowed at short wavelengths (gamma rays).
Because of the expansion of the universe, the light produced after the Big Bang should be redshifted over time, and should now appear at radio wavelengths (or more specifically, microwave wavelengths).
The Big Bang afterglow was predicted in 1948, and it was detected with a radio telescope in 1965 (resulting in a Nobel Prize). This confirmed the Big Bang theory.
This afterglow is called the Cosmic Microwave Background (CMB). It appears across the entire sky in every direction, and its spectrum is equivalent to a blackbody with a temperature of 3 degrees above absolute zero.
The galaxies are not flying apart into the universe. The universe itself is expanding. The galaxies are simply riding along as the fabric of space expands.
The expansion of the universe also causes light to get stretched to longer wavelengths, or redshifted.
So the redshifts that we measure for galaxies are not really due to their velocities away from us, but instead result from the expansion of the space itself.
Given that space is distorted by the presence of matter (and energy), it is natural that space experiences expansion along with the matter and energy.
As an analogy, imagine a 2-D universe on the surface of a sphere. Now let the sphere expand. The surface area of the sphere increases (like the volume of space in our 3-D universe), and yet there isn’t a location on that surface that is the center of the expansion, and that surface does not have an edge.
Because space is expanding along with the mass and energy, the universe does not have a center or an edge.
We can estimate the age of the universe from its rate of expansion, which is measured in the Hubble Law. If the universe expanded at a constant rate since the Big Bang, then the age of the universe would D/Vfor any point along the Hubble Law, which produces an age of about 13 billion years.
According to the Big Bang theory, the universe has not existed forever, and instead has a finite age.
The gravity from the matter in the universe should slow the expansion of the universe over time. So the universe must have expanded faster in the past than it does now. As a result, the true age of the universe should be less than D/V, or <13 billion years. In other words, because it expanded faster in the past, the universe reached its current size more quickly than if it had been expanding at a constant rate.
Big Bang expansion
But stars in some globular clusters are at least 13 billion years old. How can some stars be older than the universe? The solution to this puzzle wasn’t found until the 1990’s, and is explained in the next lecture.
The observable universe consists of the portion of the universe that is close enough so that light from it has had enough time to reach us since the Universe was born. This region is a sphere centered on the Earth, and its boundary is called the light horizon.
The microwave background radiation that we see today was produced shortly after the Big Bang by material that was only 36 million light years from our position in the universe, and yet it took 13 billion years to arrive at our location because of the expansion of the universe.
36 million light years
after Big Bang:
CMB seen today
46 billion light years
The matter that produced the microwave photons that we see today is now 46 billion lyrs away, probably in the form of a galaxy. So the current diameter of the observable universe is 92 billion lyrs.
But how can 2 points in the universe be separated by 92 billion light years when the universe is only about 13 billion years old? The universe has expanded faster than the speed of light!
Nothing in the universe can move faster than light, but the universe itself is not restricted to this speed limit as it expands. The most rapid expansion occurred right after the Big Bang in a period called inflation. In a tiny fraction of a second, the universe grew from the size of an atom to 1 billion light years across!
Because the universe has expanded faster than the speed of light, some areas of the universe have been pulled beyond our light horizon, and are outside of our observable universe.
If we wait long enough, light will eventually reach us from some of the areas currently outside of our observable universe. But other areas are too far away for their light to ever catch up and reach us as the universe expands. Those areas will always remain outside of our observable universe.
Theories of inflation suggest that the true size of the universe may be 1026 times larger than our observable universe, or 1037 light years! And it’s still expanding, so it will grow even larger!
Timeline of Universe
By the time the universe reached an age of 400,000 years, it had expanded enough so that photons of light could travel large distances without scattering. In other words, the universe was now transparent to light. So the microwave background shows us the appearance of the universe 400,000 years after the Big Bang.
Over time, very small ripples appeared in the dark matter. Gravity caused these ripples to collapse and grow to become dense clumps.
The gravity of the dark matter clumps then attracted normal matter. The resulting clumps of normal matter eventually became clusters of galaxies.
We see the clumps of normal matter that eventually became galaxy clusters in the map of the microwave background.
Will the Universe expand forever?
Or will it stop expanding and collapse (a Big Crunch)?