HPS 0410 | Einstein for Everyone | Spring 2008 |

Back to main course page

For submission Monday March 31, Tuesday April 1.

We can use Hubble's law to arrive at a crude estimate of the age of the universe. That is, we will calculate how long ago all the galaxies were crammed into our neighborhood of space. This time will be our estimate of how long ago the big bang happened. We will assume that each galaxy has moved at a constant speed for all time, although this speed will vary from galaxy to galaxy.

We will use the value of 20 km/sec per 1,000,000 light-years for Hubble's constant.

1. (a) If a galaxy is 1,000,000 light-years away from us now, according to Hubble's law, how fast is it receding from us?

(b) A galaxy traveling at 1 km/sec will travel one light-year in 300,000 years. How long does the galaxy of (a) require to travel a light-year?

(c) How long did it take the galaxy of (a) to get to its position 1,000,000 light-years distant from us?

2. Repeat the calculation of 1. for a galaxy now 2,000,000 light years distant from us.

3. Repeat the calculation of 1. for a galaxy now 3,000,000 light-years distant from us.

The final result of 1., 2., and 3. should be the same. At the time calculated, all the matter of universe would have been compressed into our neighborhood. This is our estimate of the age of the universe, often called the "Hubble age."

4. The dynamics that drive standard relativistic cosmologies are somewhat hard to understand. It turns out that this relativistic dynamics is mimicked in several important aspects by some simple dynamical systems in Newtonian theory. Those systems consist of a quantity of matter concentrated into a point in an empty Newtonian universe. That point explodes violently throwing out fragments of matter in all directions, producing an expanding cloud of debris. In Newtonian gravitation theory, every fragment of matter exerts an attractive gravitational force on every other fragment. These attractive forces act to pull the fragments of the cloud back together, slowing the rate of expansion of the cloud of debris.

There are three different types of histories for the cloud, according to the energy of the initial explosion:

*I. Low energy explosion.* The energy of the explosion is not great
enough to overcome the attractive forces of gravitation and the cloud
collapses back onto itself under gravitational forces.

*II. High energy explosion.* The energy of the explosion is
sufficient to overcome the attractive forces of gravitation. The fragments
continue to move apart without limit. The cloud is spread more and more
thinly over time and never collapses back to a point. Only a part of the
total energy of the explosion is needed to overcome the attractive forces of
gravitation. The remainder fuels a continuing rapid expansion.

*III. Critical energy explosion.* The energy of the explosion is the
exact minimum needed to prevent recollapse. Over time all of the energy of
the explosion is used up in counteracting the attractive forces of
gravitation. The critical energy level lies exactly on the boundary between
the energies of I. and II.

(a) Which Newtonian model is associated with which relativistic cosmology?

(b) While these Newtonian models are remarkably good in mimicking the relativistic dynamics, the Newtonian models differ from the relativistic cosmologies in several very important ways. What are they?

For discussion in the recitation.

A. It may seem that Hubble's law conflicts with the basic supposition of Friedman Robertson Walker cosmology that the universe is homogeneous and isotropic in space. For Hubble's law tells us that everything is rushing away uniformly from our particular galaxy. Does not that make our galaxy some sort of special center of galactic motion, different from every other galaxy? The following calculations show that the galactic motions of Hubble's law look the same from every galaxy.

Consider (0) our galaxy and galaxies (I) 1,000,000 and (II) 2,000,000, and (III) 3,000,000 and (IV) 4,000,000 light years distant from us, all in the same direction. Compute the velocities of recession of the galaxies (I)-(IV) from us.

Now imagine that you are an observer located on galaxy (I). Recompute the velocities of recession of the other galaxies. Find that Hubble's law still holds. That means that the expansion looks the same to an observer on galaxy 1 as it does from our galaxy. It is not hard to see that the same result will hold for all observers, no matter which galaxy is their home.

(In computing these velocities, use the ordinary Newtonian rule for composing velocities.)

B. If the universe turns out to have an open geometry so that space is infinite, then all of our observations are showing us only the tiniest part of space. It is a finite fragment of an infinite expanse. Given that tiny sample, are we justified in asserting that the universe is spatially homogeneous--the same in every place? Or is this fundamental hypothesis of cosmology mere supposition?

C. Some theorists find a singularity, such as the big bang, an affront to science and feel a strong need to find reformulated theories that will eliminate them. Are singularities to be avoided or eliminated from theories if possible? Why?

D. The adoption of big bang cosmology triggered a long standing debate in theology. Should we take the big bang to vindicate the theistic claim of divine creation of the universe? Theists like to point out the similarity between the creation account in Genesis--"Let there be light."--and big bang cosmology's assertion of a finite past that was dominated by radiation as we approach the big bang. Atheists, however, reply that nowhere in big bang cosmology do we find any agent outside of space, time and matter with creative powers; we just have matter and space expanding in time.