2-3 Origin of the Universe
There is a saying:
Extraordinary claims require extraordinary evidence.
Another way to say that is, "If you're going to say something really out-there, you need to be able to back that up with a lot of proof."
The whole point of the information that follows is to support a theory that says the following:
The universe started as a small, dense, hot point.
This point contained all the matter and energy that we have today.
The point exploded outwards about 13.8 billion years ago to form the universe we see today.
That's an extraordinary claim. So, we need extraordinary evidence to back that up. What follows is a short description of several different phenomena, and they will come together to back up that claim.
Waves and the EM Spectrum
A wave is any sort of back-and-forth motion that carries energy from one place to another. A type of wave like a water wave has different parts:
Light can act like a wave, and the wavelength of this light determines what type of electromagnetic (EM) radiation it is. The many different types of EM radiation there are make up the electromagnetic spectrum.
If you hear the word "radiation" and you think all of it is dangerous... it really isn't. Ultraviolet, X-rays and gamma rays are dangerous to varying degrees, but visible light, infrared, microwaves and radio waves are, to the best of our knowledge, not harmful at all. They are all types of EM radiation -- just different wavelengths.
Spectra
If you put a gas such as hydrogen into a glass tube with electrodes at each end, then put several thousand volts across those ends, the gas will glow with a particular colour of light.
As seen before, we can separate-out colours of light into the individual colours of the rainbow that make it up.
Since hydrogen is the most basic element of all, it's worth revisiting the colours that make up the pinkish-purple of hydrogen's glow:
Out of the colours of visible light, red has the longest wavelength and violet/purple has the shortest wavelength. The colour of visible light is determined by its wavelength. The unit of measurement here is the nanometre (nm), which are one-billionths of a metre.
The Doppler Effect
This applies to all kinds of waves, but the two that will be examined here will be sound waves and light waves. Since they are both waves, they both do the same thing here.
The source of a wave is the thing that made a disturbance of some sort. If we drip a drop of water on the surface of a pond or lake, we will see a series of ripples coming out from where the drop hit the surface:
(It's a series of ripples because the surface of water "bounces" up and down a few times. This is due to "surface tension" that glues water molecules together, acting a bit like a trampoline.)
If you drop a bunch of drops of water on the same spot, you will get a lot of ripples all coming out from the same place:
This way, observers A and B see the same wavelength of water waves. However, if you moved towards the right as you dropped the drops of water on the surface, the ripples would all be coming from different sources:
The source of the waves is moving towards the right -- that is, towards B. So, observer B sees wavelengths that are shorter than they should be, and observer A sees wavelengths that are longer than they should be.
The Doppler Effect describes this phenomenon: the shifting of wavelengths because the source was moving. This video does a nice job of explaining things:
And this video, within the first few seconds, demonstrates how the Doppler Effect changes sound waves:
If a sound wave has a higher pitch (it sounds like a higher musical note), its wavelength is shorter.
Putting It All Together
When Edwin Hubble looked at different galaxies in the 1920s, he found the following things:
All galaxies had the same pattern of spectral lines.
However, these lines tend to be shifted towards the red end of the rainbow.
Lines for some galaxies were shifted a little, but some were shifted a lot.
Each of these observations tell a different piece of the story.
#1: Spectral Lines
What this means is that all galaxies are made of the same elements. When you look up at the night sky and you see a star, or planet, or meteor... it's all the same stuff. There aren't any "exotic" elements out in space that aren't present on Earth.
These observations also tell us that most of the mass of the universe is hydrogen. Most stars -- and therefore galaxies -- are about 75% hydrogen, 25% helium, and traces of other elements like oxygen, carbon, iron, and so on.
#2: Redshift
All hydrogen colour-lines should have about the same wavelength: about 656 nm, 486 nm, 434 nm and 410 nm. But when looking at most galaxies, Hubble found that the wavelengths were too long: they were shifted towards the red end of the spectrum. This is redshift.
The Doppler Effect tells us that, if wavelengths are too long, the source of those waves is moving away from us. Since the sources of this light are galaxies, that means galaxies are (almost) all moving away from us.
The "almost" comes from the fact that Andromeda, the closest full-sized galaxy to us, has light which is blueshifted: it's the opposite of redshifted. (Perhaps "purpleshifted" didn't quite sound right.) This means it's coming closer to us
#3: Different Amounts of Redshift
Some galaxies have a small amount of redshift. As it turns out, these are the galaxies closest to us, and are moving away the slowest.
The galaxies that have more redshift are the ones that are farther away from us, and are moving away the fastest.
If you blow up a balloon a little bit and put dots on it with a marker, or put stickers on it -- with each dot or sticker representing a galaxy -- the dots/stickers do the exact same thing. Dots that started off closer together move away slower, and dots that were farthest-away from each other in the beginning move away from each other faster.
This means the expanding-balloon model is a good one to describe what's going on with the universe: the universe is expanding.
In the above pictures/animations, individual galaxies are yellow dots, and the photons (light waves) are shown being redshifted as the universe expands.
Putting Together a Model
Here's what we know for sure:
There are galaxies other than the Milky Way.
These galaxies are moving away from us, and each other.
We can represent this with a comic strip type of illustration, broken up into four panels:
If we go forwards in time, based on what we see now, in the future these galaxies will be farther apart.
Now let's run the clock backwards. If these galaxies are moving away from each other, that means in the past they must have been closer together.
Here comes the tricky part. If we extend this thinking further, that suggests there was one point in history at which the entire universe fit together into a single point.
What was this point like?
When you compress a gas, it gets hot. So this point must have been incredibly hot.
The volume of this point must have been very small. So this point must have been incredibly dense.
The Big Bang
If you run that model forwards from the beginning, you get a tremendous outwards explosion. Scientists have nicknamed this The Big Bang.
The Big Bang created the universe, and everything in it:
all the matter there is
the space that contains all the matter
all the energy there is
any sense of time
Based on the rate at which the universe is expanding, and the size of the universe currently, scientists have estimated that the Big Bang happend about 13.7 billion years ago.
This video provides a nice summary, and goes farther:
If you want to go a lot farther, and look at more of the evidence that supports the Big Bang, try this video:
Practice
The Basics
What wavelength of visible light is the shortest? Which is the longest?
When light from a galaxy is redshifted, what does that mean the galaxy is doing?
In the Big Bang model, what was the very early universe like?
Extensions
Andromeda's light is blueshifted. What does that mean for the future of it, and the Milky Way?
If the beginning of the universe was very hot, what does the future hold for it?
Do some internet research to find out about the Cosmic Microwave Background radiation. How does this support the Big Bang model?