Posted by Alright; this is a question I’ve been putting off for various poor reasons, but Starts With A Bang! reader Andy asks:
If Im looking at something, the light from which has taken 15 billion years to get to me, and there was only an opaque ball of radiation and stuff 15 billion years ago, why do I see formed galaxies? Shouldnt the age of the universe be: TIME LIGHT FROM OBJECT TAKES TO REACH ME + TIME TAKEN TO FORM OBJECT IM LOOKING AT?
In other words, how can I see things like galaxies that are 15 billion light years away, if the Universe isn’t even 15 billion years old?! This is a damned good question, and something that took me about two years in graduate school to figure out the answer to.
First off, how old is the Universe? Well, you can take a look at the oldest stars that we see, and you know the Universe has to be at least that old. So far, of all the stars we’ve been able to accurately date, the oldest is HE 1523-0901, coming in at 13.2 billion years old.
Image credit: SDSS.
Want to get more accurate than that? There are other methods, too, like looking at radioactive element abundances.
If we know how these elements were created, and we know their half-lives, we can figure out how old something is by measuring how much of that radioactive material is left. That’s how we know that the oldest rocks on Earth are 3.8 billion years old, for example. We can apply these methods to the Milky Way, and we find that it is between 12.3 and 17.3 billion years old. But can we be more certain than that?
Yes. Because we measure the temperature of the Cosmic Microwave Background (2.725 K), and we know what the Universe is made out of today: 73% dark energy, 27% dark matter, and maybe 0.01% radiation (photons and the like). Put those together, and you can calculate how old the Universe is today, as compared to an arbitrarily high temperature, and you find that it’s between 13.5 and 13.9 billion years old: pretty accurate for my tastes!
So, now we know how old the Universe is. Does that mean that it’s 13.7 billion light-years in size? Surprisingly, no. Take a look at the “model universe” below, which is a balloon with coins (that can represent galaxies, if you like) glued onto it:
Let’s pretend that we are the quarter at the center, and we’re looking at the dime on the left. When the Universe was younger, it was smaller, and the dime was closer to us (left panel). The dime emits light at us, and the light starts traveling towards us along the balloon. But as the Universe ages (middle panel) and ages even more (right panel), the balloon expands. This means two things for us:
- the light emitted gets redshifted on its way towards us, and
- the light has to travel a longer distance to reach us than it would have were the Universe not expanding.
So when we see the light from the dime today, and someone tells you how far away it is, it’s not always easy to tell whether they mean
- how far away was it from us when the light was emitted
- how far away is it now that we observe it, or
- how long has the light been traveling towards us, and what is that time multiplied by the speed of light?
When you read a press release, the “distance” they usually (but not always) give is the third option, which is always younger than the age of the Universe times the speed-of-light. But, if you want to know how far is that object from us today, that’s the second option, and that number can be much greater, up to 46 billion light years in any direction from us.
Now, you might ask, does this mean that space is expanding faster than the speed of light? The answer, my dear friend, is yes. Take that brain-buster to your physics teacher and watch him/her go into denial; it’s awesome! (It is, of course, because the Universe expands in a very bizarre, complicated, but moreover counterintuitive way.) Then send them to my webpage and to Ned Wright’s page for the more technical explanation.
And if your brain ain’t broke yet, check out the latest Carnival of Space, where they have my post on why we need dark matter!
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February 20, 2013
