Historical Cosmologies

## The Distances to the Stars

From Herschel's catalog, the first accurate stellar parallax measurement of 0.32 seconds of arc for the star 61 Cygni was published by the F.W. Bessel in 1838.

This was followed in 1839 by Thomas Henderson's measurement of about 1 arc-second for Alpha Centauri and the 1840 measurement of Alpha Lyrae (Vega) at 0.26 arcseconds.

1 parsec is the distance a star would have to be at to have a parallactic angle of 1 arcsecond. This distance is equivalent to 3.26 light years.

By 1840, it became clear that even the closest stars were more than one million times farther away than our Sun.

The Universe was now a large place and the intrinsic energy output of the stars had to be huge in order that their light could reach us. But what has the energy source of the stars?

Now back to , who makes two mistakes:

• Newton implicitly assumes space is flat
• Newton explicitly assumes that communication time is instantaneous

Einstein Rescues Us:

• Sets the speed of light to be independent of the motion of the source provides absolute frame and finite communication time. If not, you could have this .

• Thus, Einstein gives us causality - a very important thing

• Suggests that mass causes curvature of space and which is what we call gravity. This means that time can act strangely in a strong gravitational field:

Next step is the observation of galaxy redshifts which will demand that the Universe is, in fact, expanding.

Okay so we live in curved spacetime and now you are telling me that the Universe is exanding:

Uniform Expansion of the Universe:

Hubble noticed a correlation between recessional velocity and distance. This is known as the Hubble law:

V = HD

where V is velocity (in km/s), D is distance (in megaparsecs), H is the Hubble constant (present day expansion rate of the Universe) ### The line through the data is a "best fit" linear relationship which shows that there is a linear relationship between the the velocity at which a galaxy moves away from us and its distance. This linear relatinship is consistent with a model of uniform expansion for the Universe.

This simple relation implies something remarkable about the Universe.

At some earlier time, all the galaxies had to have been together in the same space at the same time the Universe was once really small. It is important to realize that the galaxies are stuck to the surface of the universe by gravity and its the surface that expands. The galaxies themselves are not moving but travel along with the surface as shown here .

Run time backwards and realize that all the galaxies used to be together.

So what do we know now:

• Gravity: Curvature of SpaceTime: Mass causes this curvature; space is deformed around this mass; this deformation is gravity. The more mass there is, the more deformation (curvature) there is and gravitational field becomes stronger

• Galaxy Redshifts indicate that galaxies are moving radially away from one another

• V=HD demands that the expansion is uniform

• Since the universe is a SURFACE (in spacetime) then it has no CENTER!

• In the past the Universe was a smaller place but still had the same mass in it - in the way distant past, all of this mass was in the same place at the same time, except that it wasn't mass it was Energy

• and you:

Photons have an energy related mass. Photons are therefore effected by gravity. Light is bent in a strong gravitational field as the surface of the universe goes from being flat (light travels in a straight line) to curved (light follow the curved trajectory). This principle is shown in this animation .

The distribution of mass on in the universe determines the detailed shape of the surface light is constrained to follow this surface and this allows the universe to be observed!

Ants with Flashlights in curved space time

• The ants can only communicate by flashlight the photons in the flashlight must follow the curvature of the universe because of the rule and so the photons must follow the blue trajectory and not the red one

Returning now to the expansion of the Universe with the help of some handy demonstration ants:

1. Place them on the surface of a bomb and given them each a telescope. Explode the bomb. V is not related to D.

2. Place the ants on the surface of a balloon and inflated the balloon; let the ants randomly walk around the surface of the balloon V is not related to D

3. Get some glue. Glue the ants to the balloon and inflate it. The separation between ants increases and V is observed to be proportional to D. The velocity of the ants is completely determined by the expansion motion of the surface.

4. Now suppose the ants are only partially glued (one leg is glued down). The ants are thrashing around the glue spot and therfore have a random component to their motion. Hence the total motion of the ants is due both to this random component and the expansion component. For short separations, the random component could dominate over the expansion component.

Horizons and the Expansion Age of the Universe:

V = HD c = HD ==> D =c/H ==> This is our causal horizon - beyond this distance something would have to travel faster than the speed of light in order to communicate with us. All observers are surrounded by such a horizon.

Horizons are okay. Our assumption about homogeneity means that the stuff beyond the horizon is the same stuff we already know about. This assumption must be correct due to horizon overlaps and causality.

Back to the Ants glued to the balloon:

If you know the rate of inflation of the balloon (the expansion rate of the surface) and the surface area of the balloon (which is proportional to its radius) then you can determine how long it has taken for the balloon to reach its present size.

Example: I attach the balloon to a slow pump which increases the radius of the balloon by one foot each day. This is the expansion rate that I measure. I measure the balloon to have a radius of 8 feet. This means the expansion age of the balloon is 8 days.

V = Hd ==> 1/H = D/V

Distance/Velocity = Time

1/H = the expansion age of the Universe.

This is how long the Universe has been expanding. What it was doing prior to the expansion is anybody's guess. Previous Lecture Next Lecture Course Page