The Age of the Universe

The universe is expanding !!  The visible universe is made up of clusters of stars, held together in galaxies by their mutual gravitational attraction.  Our own galaxy, the Milky Way Galaxy, is a spiral galaxy.  When we look at other galaxies, they all appear to be moving away from us!!!!  This means the universe is expanding. But, how do we determine that they are moving away from us????

Doppler Effect for Light  There is a relationship for electromagnetic waves (i.e. light) that relates the frequency, wavelength, and speed.  c = frequency x wavelength  We know that light always travels with v=c. But, for a source that is moving AWAY from us, the wavelength must be LONGER.

“Red Shift”  Early in the last century, astronomers noticed that the light spectrum from ALL galaxies was shifted towards longer wavelengths – towards the RED end of the spectrum. Thus, all galaxies are moving away from us  The universe is expanding.

Hubble’s Law  In 1929, Edwin Hubble compared the amount of red shift to the distance to the galaxy. He showed that the amount of red shift (i.e. the velocity) is proportional to the distance.

How do we measure the distance?   Basically, it is pretty hard !!!  But, the point is, we know have a decent idea of these distances, to within 20-30% or so.  Thus, the Hubble constant is determined to a similar accuracy.

The Age of the Universe  Note the units (km/s/Mpc)!! This is a distance/time/distance, or simply, 1/time.  Thus, the inverse of the Hubble constant is a time. This is the age of the universe!!!! Basically, think of it that it takes this amount of time for light from the oldest stars to get to us. The oldest stars are the farthest from us.

Fate of the universe?  Will the universe keep expanding forever?  Will it eventually collapse in on itself?  The answer to this question is still being debated – but it depends on how much matter there is in the universe ….  But, nonetheless, it appears that there is very close to being exactly the right amount of “mass” in the universe !!  Still, this leads to some BIG problems!!!!

Dark Matter  Our galaxy does not make sense!  If we look at the way that it rotates, it only makes sense if there is a spherical distribution of matter, which is not what we see.  Thus, we infer that there must be some DARK MATTER that we cannot see.

The density parameter  If there is more mass than necessary, i.e. the universe will stop expanding, and start contracting, we say the universe is closed.  If there is less than is necessary, we say it is open.  If it is exactly right, we say it is flat.  This is typically expressed in terms of the density parameter, which is the ratio of actual mass to required mass for flatness.  Closed – density parameter >1, open – density parameter 1, open – density parameter <1, flat – density parameter =1.  Based on our most recent data, the universe is very close to being FLAT.

Dark Energy  Recent observations have lead to the conclusion that the universe is not just expanding at constant velocity, but it is actually accelerating.  It cannot be mass that causes this – that would slow it down. It must be something that is “pushing” it outwards.  We call this DARK ENERGY.

So, what is the universe made of?  Atoms, nuclei, the normal matter that we see every day – 4%  Dark Matter – 23%  Dark Energy – 73% Conclusion: 96% of the universe is made of stuff that we have never observed !!!!!!!!!!!!!