How Do We Know Normal Matter Makes Up 5% of the Universe?
As I write this, I’m watching the Illinois/Ohio State game. Currently, the game is tied 14/14 and I am quite pleased. I mention this not because you may be a football fan but as a hint to where my new job is. One of these schools will be my new professional home.
Now, on to our regularly scheduled post.
Here is a complete inventory of the universe. It is nothing short of amazing that I can write this down with any reasonable level of confidence. This is a great time to be in cosmology.
The universe is made up of:
- 5% normal matter. The is everything we currently see and interact with and is made up of protons, neutrons and electrons.
- 25% dark matter. This is composed of an unseen and undiscovered particle. It is probably cold, has no electrical charge and has lots of mass, hilariously dubbed a Weakly Interacting Massive Particle (WIMP).
- 70% dark energy. Not much is known here except that this stuff is more or less evenly distributed around the universe and, if it exists, accounts for the acceleration of the universe.
It is an excellent question to ask, “How do we know that only 5% of the universe is composed of protons, neutrons, electrons, neutrinos, etc?”
One way is to look at thousands of galaxies, measure the rate at which they rotate, then infer the mass of each galaxy. Add all that up and you get an answer of close to 5%,
Of course, one could easily miss some and this doesn’t account for stuff not in galaxies so this is probably an underestimate.
A much more accurate method is to look at the matter created during primordial nucleosynthesis:
In the time period between about 100 seconds and 30 minutes after the Big Bang, but mostly with the first three minutes, the temperature and density of the universe were appropriate for the efficient synthesis of the light elements. Just as in the Sun, some of the hydrogen nuclei were underwent fusion to form heavier nuclei including helium nuclei. This was the era of primordial nucleosynthesis. The current abundances of the light elements reflect what occurred during the epoch of primordial nucleosynthesis and therefore place strong constraints on the state of the universe and the baryon density during that time. (Full article)
During this time, primordial nucleosynthesis predicts that during this window of time in the history of the universe, there should be a certain abundance of Helium, a little less, but very well-defined amount of Deuterium, even less Lithium and smaller amounts still of Helium-3.
The important thing is these very specific elements and their relative amounts are well-predicted and defined. If what we think about the Big Bang and the large scale structure of the universe is correct, then we should be able to find these elements in their correct abundances when we look at pristine areas of the universe.
By pristine, I mean areas of the sky that have not been contaminated with clusters of galaxies, stars, supernovae, or an interstellar medium.
So, the argument goes like this: if the universe was dominated by radiation in its earliest moments, and if the universe expanded at the rate we think it did, then we should see 25% helium nuclei, and correspondingly smaller amounts of Deuterium, Lithium and Helium-3.
The abundances of these elements we see when we look at unspoiled parts of the universe is exactly that.
Nucleosynthesis provides a direct measurement of the total amount of normal matter in the universe. The diagram above shows how the measurement is made:
- Pick an area of the sky where no supernovae have occurred and no galaxy clusters.
- Look at the light from a distant light source on the other side of that region.
- Measure the light traveling through that space and note the properties of the elements in that region.
- The abundances of the elements found there should correspond to what we think the early universe was like.
Technorati Tags: cosmology, dark energy, dark matter
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2 opinions for How Do We Know Normal Matter Makes Up 5% of the Universe?
Frederica
Nov 20, 2007 at 2:27 pm
Tony, I hope this question is relevant here:
I recently have heard of -and read about- a “huge hole in the universe”… some kind of void that is nearly a billion light years across, nothing in it, nada, zip. Not even dark matter according to the writers.
I would have a hard time believing you haven’t heard of this, I’d like to know what is your input on that ? Why haven’t you brought this up ? Is it cosmologic bullsh!t ?
here’s a link amongst few :
http://www.nrao.edu/pr/2007/coldspot/
r06u3AP
Dec 9, 2007 at 9:22 pm
That analysis is fine for the relative abundances of the light elements, but when you say “X% of the Universe”, what is that a percentage of? To say that the amount of “normal” matter is a fraction of something else, you have to define that something else first, don’t you?
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