How old the Universe is?

The Expanding Universe

In the 1920’s Edwin Hubble, having abandoned a boxing career (one exhibition vs French champion), turned his blackened eyes toward galaxies. At the time there was debate over whether these foggy ‘nebulae’ were within our Milky Way galaxy or  other entire galaxies at extreme distances. Aristarchus’ method to measure the distance to the sun won’t work because there is no baseline big enough to notice any parallax in the position of galaxies. The lack of parallax shows that they are stinking far away but leaves no way to put a number on it.

Hubble used a clever trick not of his own invention. There is a type of star called a Cepheid variable star. One of a class of stars whose brightness varies at a steady rate. The rate at which they vary though depends on how bright they are. Not how bright they LOOK to us which also depends on how far away they are but on how bright they really are - intrinsic brightness. So? So, let’s say you can find one of these suckers within another galaxy like the Andromeda galaxy. You measure its rate of oscillation which you can easily do with a clock, telling you how bright it really is. Then you measure how bright it looks from here telling you how far away it is.

Brilliant!

Now Hubble had a way to start to categorize distances to the smudges in the sky - other galaxies! And they turned out to be really stinking far away as in millions of light years. But he didn’t stop there. As long as you are staring at these galaxies (with the new 200 inch telescope on Mount Palomar) you might as well capture their spectrum as well. When Hubble did this he found the usual spectral lines for hydrogen and helium but the lines were shifted more to the red end of the spectrum.
Moreover as his data table grew, he found that the farther away a galaxy was the more red shifted it was. What the heck?

The Doppler Effect

The Doppler effect is a fairly common phenomenon. Think about what a race car sounds like as it passes you. Or a fire engine siren. There is a drop in pitch as it passes you. The sound waves are crushed together as the source comes toward you and stretched apart as it recedes from you causing that Mrrrrrrroooooowwwww sound! The same happens with light. Crushing light waves together by a source coming at you would make them look bluer (shorter wavelengths) than they really are and stretching them out by a source moving away from you would make them look redder (longer wavelength)  than they really are. AND, of course, the faster the motion  the more the stretching/compressing.

So, the farther away the galaxies the more the light is shifted to the red means the faster those galaxies are moving away from us. Hubble detected virtually no galaxies being blue shifted or moving toward us.34

The geometry of the universe then is billions of galaxies all moving away from each other and the farther out ones moving faster. That is exactly the geometry of an explosion!

(Note that the motion of galaxies is actually the stretching of space  itself as opposed to galaxies moving through space. Think raisins moving away from each other as the bread rises. )

The Hubble Constant

The Hubble constant is a measure of how much faster for how much farther away. It’s current value is

H = 70.0 km/sec/Mpc 5

Wha??? 70 km/sec is a speed. Mpc is a distance called mega parsec. One million parsecs and a parsec is about 3 light years. In simple terms then, a galaxy 1 mega parsec away would be moving away from us at 70 km/s. One that is 2 mega parsecs away would be moving at
140 km/s . ...And so on.

So what?

So what is that if everything is moving away from everything else that means that everything was closer together yesterday than it is today! Go further back in time and everything must have been very close together. Go all the way back and you arrive at the big bang. The zero moment when the entire universe burst into existence! When was that?

Take any galaxy then and ask how long it took it to get where it is today. Take one that is going 70 km/s. How long does it take to get 1Mpc away at that speed. Well this is the first week of Physics 1. Train A is going 50 mph. How long does it take it to go 200 miles?

200miles/50miles/h = 4 hours

in our case we have a problem with speed being in km/s and distance being in Mpc but the calculation is the same. Distance/Speed gives you time. Sparing you the uniit conversions when we do that Physics 1 calculation we get about 13 billion years.

I’m stunned by this. A simple high school calculation leads us to a measure of the age of the universe. Clever we are!

The microwave background

In 1965 two Ball Labs scientists, Penzias and Wilson, were using a giant microwave horn to detect radio emissions from regions of the sky. Strangely, there was a background ‘hum’ to all the data that they couldn’t account for. They suspected something in the wiring, the local power lines, and even the pigeon poop in their horn (which they had to scrape out to eliminate from possible errors). The hum remained. It is was pervasive. It seemed to be coming from every direction in the sky.

Now what could this be? Assume there was a big bang to start the universe. There would be have been some very energetic photons (light) released just after that moment. As that light travels through an expanding universe though it gets ever more red shifted to longer and longer wavelengths, past red, past infra red all the way down to microwaves (radio). So looking at the wavelength of this microwave background you can calculate backwards to how old the universe is and you get . . .

about 13 billion years!

Bang . . . Big Bang!

Two completely independent measurements leading to the same number. That was the slam dunk convincing people of the truth of the Big Bang and the expanding universe. Work remains of course working out details but the general idea of a universe that had definite beginning and that it was about 13 billion years ago6 is no longer questioned by those in the field.