Starts With A Bang! » Q & A http://startswithabang.com Ethan Siegel's blog/video blog about Cosmology, the Universe, and everything else Sat, 04 Apr 2009 20:12:38 +0000 http://wordpress.org/?v=2.3.2 en Gravitational Waves: Inflation or not? http://startswithabang.com/?p=1801 http://startswithabang.com/?p=1801#comments Thu, 17 Apr 2008 19:23:38 +0000 ethan http://startswithabang.com/?p=1801 Nothing gets past you, does it? A scientific paper came out earlier this week, and I took a look at it, sighed, and Jamie asked me, “What?” And I said to her, “When I see bad science, it just makes me a little bit frustrated and sad.” Of course, I had no intention to write about it.

But then Starts With A Bang reader Matt emailed me, and writes the following about this press release that he had seen:

You have two explanations for these gravitational waves now and that much I understand. But they make it sound as if symmetry breaking and inflation are competing theories. They aren’t, right? Do phase transitions influence inflation (it would make sense)? How are those two related? The inflation rate depends on the energy density of the universe (-> scalar fields), right?

And the point is: Even if we attribute the gravitational waves to the process of symmetry breaking, we’d still need to explain the uniformity of the universe because symmetry breaking only explains the origin of the fundamental forces.

So the paper is by Lawrence Krauss, whom I met once back in 2006, when I was giving a talk at Vanderbilt. Lawrence shows up about 40 minutes late (to my one hour talk), makes a scene when he walks in, and demands, “What did I miss?” Feeling indulgent, I gave him a 15 second synopsis of the last 40 minutes, and he goes, with a satisfied smile, “Oh, not much then.” Way to make a good impression on me, Lawrence.

Anyway, his scientific paper doesn’t have anything wrong with it. He basically talks about how a global phase transition can generate gravitational waves, which is 16 year-old news, and those waves might be strong enough to show up in the CMB, just like those from inflation. Is this big news? Come on, anyone can write a paper where you make gravitational waves (the link is to a paper I wrote in 2005).

Here is the important difference, however:

  • Inflation predicts a scale-invariant spectrum.
  • Other mechanisms to make gravitational waves don’t.

A “scale-invariant” spectrum means that energy is evenly distributed in waves of different sizes. Let’s compare the spectrum of inflation (green curve):

to the spectrum in Lawrence’s paper (figure from the paper; he plots things in different units):

and just for fun, let’s throw in the spectrum that my old paper predicts (it’s very different from inflation):

Now, here’s the thing missing from Lawrence’s paper (and admittedly, my paper, too). What is this going to look like in the Cosmic Microwave Background? People have computed it for inflationary models, and know that the shape of the curve should look just like this (the blue curves are for different amplitudes of inflationary models),

so people can go out and try to measure it. Specifically, for those of you who want details, this is looking at the B-mode spectrum of the microwave background, which is one of the things that Planck is designed to measure. What does this new paper predict for their data? Well, they conveniently don’t publish it. Why not? Because it would decidedly be very different from anything resulting from inflation.

Lawrence’s paper talks about something that happens way after the end of inflation, and doesn’t affect the spectrum from inflation or anything related to inflation at all. The paper just gives an extra way to generate gravitational waves of large-enough amplitude that they might show up in the CMB. And they might, if the new physics which he made up is correct. Which, who knows, it might be, and at least we have something new to look for. But this research does nothing to eliminate the need for inflation or change the predictions of inflation, and the press release is indeed wrong for implying that. Thanks, Matt, for forcing me to clear that up.

It’s getting near the end of the week again, so check out this week’s Carnival of Space over at KYsat. Is the KY for the state or the… other thing?

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Why Explore Space/the Universe? http://startswithabang.com/?p=1793 http://startswithabang.com/?p=1793#comments Fri, 11 Apr 2008 18:18:54 +0000 ethan http://startswithabang.com/?p=1793 Fraser Cain over at Universe Today sent out a question to the Astronomy/Astrophysics/Space communication community today. And he asks:

Why should we spend our time/money/resources on exploring space when there are so many problems here on Earth?

This is something that, for better or worse, I had a knee-jerk reaction to. Here’s what I wrote back to him:

This is like asking why we should spend money on making our city better when there are so many problems here in our own homes. Or why we should spend money on understanding our whole world when there are so many problems here in our own country. Space is something that we are not only a part of, but that encompasses and affects all of us. Learning about the grandest scales of our lives — about the things that are larger than us and will go on relatively unaffected by whatever we do — that has value! And it might not have a value that I can put a price tag on, but in terms of unifying everyone, from people in my city to people in a foreign country to people or intelligences on other planets or in other galaxies, space exploration is something that is the great equalizer. And the knowledge, beauty, and understanding that we get from it is something that one person, group, or nation doesn’t get to keep to itself; what we learn about the Universe can be, should be, and if we do our jobs right, will be equally available to everyone, everywhere. This is where our entire world came from, and this is the abyss our entire world will eventually return to. And learning about that, exploring that, and gaining even a small understanding of that, has the ability to give us a perspective that we can never gain just by looking insularly around our little blue rock.

What are your thoughts on the matter? Is this valuable, or am I just being completely naive and idealistic in my views of the value that understanding the world and Universe around us can bring? Whatever you think, you can read what the other responders had to say here.

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Energy Conservation and an Expanding Universe http://startswithabang.com/?p=1792 http://startswithabang.com/?p=1792#comments Fri, 11 Apr 2008 14:32:57 +0000 ethan http://startswithabang.com/?p=1792 So, what’s the deal with this one? startswithabang.com reader Scott Stuart asks the following question:

I was reading “The First Three Minutes” last night and came across an
interesting section about blackbody radiation and energy density. The
author states that as the universe expands, the number of photons
running around (in the CMB, for example) is unchanged, but their
wavelengths get stretched. The energy in a photon is, of course,
inversely proportional to its wavelength, so the energy content of a
photon decreases as its wavelength increases. That seems to mean that
the total energy content of the photons decreases due to the expansion
of space. Now the energy density clearly decreases as the volume
increases, but this argument says that the total energy decreases as
well. Does that mean that the expansion of space is not conserving
energy? Or is the energy “going” somewhere?

Remember the law of conservation of energy? It states that energy can neither be created nor destroyed, only transformed from one form into another. Now Scott asks how this works in an expanding Universe, because quite clearly the rules change!

His point is that if I have a bunch of photons in my Universe, and I stretch my Universe, the photons will change wavelength to accomodate the change in the size of the box. So if I double the size of the Universe, the energy in photons in the Universe halves.

What about matter? Both normal matter and dark matter don’t change their mass as the Universe expands, so that seems okay. But what about the energy in the gravitational field? After all, there is such a thing as binding energy, and as I increase the distance between objects, the gravitational binding energy (which is a form of negative energy) goes up (or closer to zero). Unfortunately, we don’t have an exact definition of gravitational field energy, so that gets sticky.

Now let’s throw dark energy in, and make the conundrum worse. All of the evidence for dark energy (currently) points towards it having a constant energy density. This means that as the Universe expands, and we wind up with more space, we are constantly creating more and more energy. So what’s the deal? Is energy conserved, or isn’t it?

If we define energy like we’re used to defining it, that is, locally, the answer is no. If we take the energy density of the Universe and multiply it by the “volume” of the Universe, we get a number for total energy that changes over time. Just after the big bang, most of the energy density was in radiation, which decreases faster than the volume increases due to the stretching talked about above. So at the start, it looks like the total energy of the Universe is dropping. Then the Universe becomes matter dominated, and then energy appears to be conserved, since the product of the matter’s energy density times the volume stays constant. But then the matter density drops below the dark energy density, and now, the Universe is dominated by dark energy. The product of the dark energy density (which is constant) times the volume (which is increasing) is increasing! So it looks like the total energy of the Universe first decreases, then becomes fairly constant, and then increases again.

How is this possible? The problem, as I’ve already alluded to, is that energy is only defined locally. That means that we have no idea how to define something like “the energy of spacetime” or “the energy of the Universe’s expansion.” Without that, all we can do is state the rules for how energies and energy densities change as the Universe expands and ages.

Maybe you don’t like my answer to this question. In that case, you can try Sean Carroll’s answer, or read Steve Carlip’s answer (the third one down). The big problem is that we don’t know how to define gravitational energy on cosmological scales. Clearly, there’s a lot of it! Maybe one interesting thing to do would be to define it in the one unique way that would conserve total energy, and to learn what that is? Then, perhaps, we can test it?

Thanks to Scott for a very tough, but very good question! You have one? Send it in!

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Why doesn’t Light Age? http://startswithabang.com/?p=1781 http://startswithabang.com/?p=1781#comments Wed, 02 Apr 2008 09:05:12 +0000 ethan http://startswithabang.com/?p=1781 There’s a graduate student that I’m sort-of mentoring/working with at Arizona, named Xiaoying Xu (hi Xiao!). She’s bright and curious, and she asks some very good questions. She asked me one yesterday that’s pretty tough to wrap your head around:

How do I explain to someone why light doesn’t age?

Well, here on Earth, time progresses at a certain speed. That is, if I measure how many seconds tick by as the Earth revolves once around the Sun, I’ll get 31,556,926 seconds. (31,558,150 if I’m measuring a sidereal year.)

But let’s say in the course of that year, I put you on a rocket ship, and you come back exactly one Earth revolution later. I send you off at 12:00 AM on New Year’s day. Well, if you’re moving at typical rocket ship or satellite speeds (a few kilometers/second), your clock will be about one-hundredth of a second faster after a year due to the time dilation effect of special relativity, hardly noticeable.

Big deal. But what if you start moving fast? If you move at 10% the speed of light (30,000 km/s), your clock will say that it’s about 44 hours earlier than mine. While I ring in the New Year, you think it’s 4 AM on December 30th. If you get up to 90% the speed of light, I get a kiss and champagne while you think it’s early morning on June 9th. At 99.99% the speed of light, only 5 days will have passed for you while I’ve lived a whole year. And at 99.9999999% the speed of light, an entire year passes for me in just 23.5 minutes for you.

So as things move faster, time passes slower for them. Now, being made of matter (and having mass), we can never move at the speed of light. But things that do travel at the speed of light never have any time pass for them. Now as if that wasn’t neat enough, most things that we know of will decay eventually. Things that we like, such as neutrons. But if they move faster, they live longer. That’s how cosmic rays known as muons can actually reach the surface of the Earth, because they move at 99.999% or more the speed of light! But what about light? Well, that’s the one thing in the Universe that we know will never decay. Protons might decay (we know that if they do, their half life is over 1035 years), electrons might decay, but particles of light can’t. Because time doesn’t pass for them!

And that is why light doesn’t age.

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Weekend Diversion: Rainbows! http://startswithabang.com/?p=1759 http://startswithabang.com/?p=1759#comments Sat, 15 Mar 2008 09:05:56 +0000 ethan http://startswithabang.com/?p=1759 Starts with a bang reader Zrinka asks us how rainbows work, and that’s a great question for the weekend, since I’m driving up to Portland, OR right now. (The desert is lousy for rainbows when it doesn’t rain!) So you’ve seen something like this before, although maybe yours isn’t as famous as Galen Rowell’s:

So how do you make one? Well, this works just like light passing through a prism will separate into its colors (right), because the longer wavelength light travels faster in any medium. So red light has a longer wavelength than purple, and so not only travels faster through glass or water than purple light does, but also bends by a smaller angle than purple light does.

So if the Sun is behind me and there are drops of water in front of me, the sunlight can come in to the raindrops, get reflected off of the back of the water drop, and come back to my eye, except light of different wavelength comes out at different angles. The image below shows you the difference between red light (which comes out at 42 degrees) and purple light (which comes out at 40 degrees):

So that’s why red appears on the outside, because it makes an arc of 42 degrees, and purple appears on the inside, with an arc of 40 degrees. Now sometimes, if you look closely on a nice bright day, you’ll see a second rainbow outside the first, with the colors reversed! This is what happens when you get a second reflection happening inside some of the drops, and you might see something like this!

So that’s how rainbows work; now the next time you see one, you’ll know that where it appears is because of your position relative to the sun and the rain, and the shape and arc-size of the rainbow(s) are the same wherever you are!

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Q & A: Where does Matter Come From? http://startswithabang.com/?p=1750 http://startswithabang.com/?p=1750#comments Tue, 04 Mar 2008 19:56:29 +0000 ethan http://startswithabang.com/?p=1750 I love The Straight Dope. For 35 years, people have written in and asked some of the most difficult-to-answer questions on any topic you can think of; the staffers, writing under the pseudonym Cecil Adams, do their best to get to the bottom of their questions. Well, they also have a message board, and I saw one of the most difficult questions I’ve ever seen there:

Where does all the matter in the universe come from?

I’m no[t an] astrophysicist but I understand a little about the Big Bang Theory and also that there’s lots of stuff we don’t know or probably ever will know about it.

But the universe is awfully big and must have an awful lot of matter in the form of asteroids, stars, asteroids and suchlike. Did all matter in the universe originally exist at the centre of the Big Bang or is new matter being constantly created? If so, how?

None of the responses up there even begin to do this question justice, so let’s take a look ourselves. First off, there are two possible interpretations of this question, and I need to choose which one to answer. Is the question asking:

  1. Why is the Universe full of stuff? That is, why is there anything with any energy at all instead of nothing? Or…
  2. Why is the Universe full of matter? Energy could take any shape or form, but why matter, and how did it get there?

I’m assuming the second one (although if someone wants to ask the first, I’ll give it a shot); Why is the Universe full of normal matter? This isn’t something we expect, mind you. Here’s what we know as normal matter:

protons, neutrons, and electrons make up all the planets, stars, gas, and dust that we know and observe in the Universe. But the problem is, whenever we go into a laboratory and try to make some of this normal matter, for every particle of normal matter we make, we also make one of antimatter. But the Universe as we know it is made up almost exclusively of matter, with almost no antimatter. Every galaxy we see is matter, not antimatter. Every cloud of gas and dust we see is matter, not antimatter.

Why? If we take a look at wikipedia’s Unsolved Problems in Physics article, this is the second one on the list. But there is a whole bunch we do know about it, even if we don’t know the entire story.

First off, and this is the first rule of any scientific inquiry, is no cop-outs. That means, we would never just say, “Oh, it had to be there from the start of the big bang.” No; we want to figure it out, so we want to start with equal amounts of matter and anti-matter, and see if we can make more of one type than the other, naturally. We call this process Baryogenesis (other articles here and here).

So what do we need to make it happen? We need three things, known as the Sakharov conditions:

  1. You need an interaction that violates Baryon number conservation, which means you need to be able to make more protons than antiprotons or something akin to that.
  2. You need to violate CP-symmetry, which means (in English) that you need particles and antiparticles to decay into their various products at different rates.
  3. You need to be out of thermal equilibrium.

So, how can we do that? Let’s give you my favorite example; let’s assume that at some high enough energy, there are superheavy particles called X. The X has a charge +4/3, and there’s also the anti-X, with charge -4/3. When the Universe is very hot, it can be stable, and you make equal numbers of X and anti-X particles. Now, these X and anti-X particles aren’t stable, and they decay. Maybe the X-particles decay like this:

And make a positron and an anti-down quark (1/3 of an anti-baryon). Or maybe they decay into two up quarks instead (2/3 of a baryon). But let’s say the anti-X-particles also decay (because decays happen when the Universe cools and becomes unstable — that’s condition 3): they can decay into electrons and a down quark (1/3 of a baryon), or two anti-up quarks (2/3 of an anti-baryon). But what if the X decays into 49% positrons and anti-downs and into 51% two ups, and the anti-X decays into 51% electrons and downs and 49% two anti-ups? Well, that’s what can happen if you violate CP-symmetry (that’s condition two). Let’s put it all together and see, at the end of the day, what are you left with? A bunch of particles and antiparticles that will find each other and annihilate (down/antidown, up/antiup, and electron/positron), but then you’ll have that 2% left over! And what is that 2% made up of? Electrons, down quarks, and up quarks — just the stuff you need to make normal matter! And so you meet all three of these conditions, and just like that, you can make more matter than antimatter, starting with equal amounts of both!

Now, we aren’t sure that this is how it happens, nor are we sure that any of these related methods is how it happens either. But we can pretty definitively say that the Universe is made up of matter and not antimatter, and that there are a number of physical ways to make more matter than antimatter in the Universe. It isn’t being created now, nor is it fair to just assume it was created at the Big Bang, but it looks like we can make it rather shortly after the Big Bang, and pretty much all in one go. And that tiny little asymmetry, the little extra bit of matter that was created as opposed to antimatter, makes up and gives rise to everything (except the Microwave Background) that we see today. Pretty neat!

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How old is the Sun in Galactic years? http://startswithabang.com/?p=1744 http://startswithabang.com/?p=1744#comments Wed, 27 Feb 2008 21:23:36 +0000 ethan http://startswithabang.com/?p=1744 The Moon goes around the Earth, the Earth goes around the Sun, and the Sun goes around the center of the Milky Way. We know the Moon takes about 4 weeks to make its trip around the Earth, and that causes the Moon phases:

We also know that the Earth takes one year to go around the Sun, and that causes the seasons:

We also know that the Earth has been around for about 4.5 billion years, which means it has gone around the Sun about 4.5 billion times. Well, now I ask the question(s):

How long does it take the Sun to go around the Milky Way? How many times has it done that so far, and how many times will this happen before the Sun finally dies?

Well, we know that we travel in (roughly) a circle around the center of the Milky Way, and that our radius from the center is about 8 kiloparsecs, or roughly 26,000 light years. That means our Solar System (including the Sun) needs to travel a distance of 1.55 x 1018 kilometers to go around the Milky Way once. If we know how fast the Sun is moving, we can figure out how long a Galactic Year is. Well, we can both measure and calculate its velocity to be 220 kilometers/second, and so we can just do the math, knowing that there are 31,556,952 seconds in a Gregorian Year, and we find that it takes about 223 million years to make one galactic year.

So, if the Sun is 4.5 billion years old, that makes it about 20 galactic years old. If the Sun has a total lifetime of around 10 billion years, then it has a total galactic age of around 42 galactic years.

What? Did I just say the answer is 42?! Well, this means that one possible question is “What is the Sun’s lifetime in Galactic Years?”

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Q & A: Why is the Microwave Background so Uniform? http://startswithabang.com/?p=1742 http://startswithabang.com/?p=1742#comments Tue, 26 Feb 2008 17:41:33 +0000 ethan http://startswithabang.com/?p=1742 startswithabang.com reader Andy has a great follow-up to his question on the Age & Size of the Universe, and asks:

why does the CBR “appear” to come from a light sphere that “appears” NOW to be larger than the universe WAS when it first set off in a straight line on its 13.4 billion year trip???

The “CBR” stands for “Cosmic Background Radiation,” and it refers to the (presently) microwave background. Here’s why Andy’s question is actually profound, and was known for about 20 years as either the homogeneity problem or the horizon problem. The problem is that, when we look up at the sky, and take a look at the microwave radiation (that’s the leftover radiation from the big bang), we find that it’s the same temperature, 2.725 Kelvin, in every direction (top image at left). Arno Penzias and Robert Wilson won the Nobel Prize for discovering this uniform microwave background.

How uniform is it? If we subtract off 2.725 Kelvin from the entire image, we can measure the deviations from the mean temperature. What we find is that there is a range from -0.004 (blue) to +0.004 (red) Kelvin deviating from that mean temperature (middle image at left). What causes that? The motion of us against the microwave background, or a doppler shift from our local velocity. The discovery of this was also worth a Nobel Prize, this time for George Smoot and John Mather, who co-discovered it.

More recently, we’ve discovered that if you take away that doppler shift as well, you still find hot and cold spots, but these are only about 0.00003 Kelvin in range from coldest to hottest (see the lower image at left).

This is the horizon/homogeneity problem: Why is the temperature of the microwave background so uniform in all directions? This is really hard; the Universe is 13.7 billion years old, but was only about 380,000 years old when the background radiation was emitted. If different regions of the sky are now separated by up to 93 billion light years, how is it that they have temperatures that are so close to being equal to one another? After all, they’re so far away that they couldn’t have been in contact with one another, so they wouldn’t have a chance to thermalize, or achieve the same temperature. Look at the graphical illustration of this that I stole from UC Santa Cruz:

What the above image shows is that there are many different causally disconnected regions of space, that somehow have the same properties (e.g., energy density). If you ask how many, the answer is in the thousands. How do we solve this problem?

The only reasonable solution is to state that, somehow, these regions must have been causally connected at one time in the past, otherwise there’s no way for them to have the same properties. How is this possible? The Universe needed to expand, early on, by an incredible amount. What we can imagine is, just like we have “dark energy” now, that causes the Universe to expand without slowing down, we could have had it when the Universe was very young, only much stronger. This means that the Universe, in only a fraction of a second, could expand by an exponential amount, effectively increasing its size by a huge factor, like 10100! Even if it started out only as the size of a proton, this theory, known as inflation, says that the entire Universe could expand to be trillions and trillions of light-years in size, practically in an instant. So if you have this inflationary period near the beginning of your Universe,

you then end up with a Universe that should have the same physical properties everywhere, since everything was in contact with everything else prior to inflation. Does it sound crazy, or plausible, or annoying? There’s other evidence for it too, but this was one of the biggest arguments for it, and when Alan Guth discovered it, his paper was entitled “Inflationary Universe: A possible solution to the horizon and flatness problems,” a testament to how good Andy’s question really is!

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Q & A: What is Energy? http://startswithabang.com/?p=1741 http://startswithabang.com/?p=1741#comments Mon, 25 Feb 2008 18:07:43 +0000 ethan http://startswithabang.com/?p=1741 Let me set the scene for you: I’m fresh off my Ph.D., teaching introductory physics at the University of Wisconsin. I’m trying to demonstrate how to turn potential energy into kinetic energy, and so I ask this simple question:

What is Energy?

And I get a stunned silence back from the room. One of those silences where 25 faces look back at you with eyes that say, “no, you’re the one teaching us; you need to answer that one!”

And I confess, this is one of those questions that’s looks like the easiest thing in the world, and yet there’s no good answer for it. Put simply, energy is possibly the most basic thing in the whole Universe. We know a whole lot about it:

  • we know that all mass and matter contains it,
  • we know how to quantify it,
  • we know how much is stored electrically, chemically, thermally, sonically, etc.,
  • we know how to convert it from one form to another,
  • we know how to use it to accomplish things (i.e., to do work),
  • we think it can never be created nor destroyed,
  • and we can generate, calculate, and measure its various forms.

So at the end of the day, we can identify it, make it, and do stuff with it, which makes it incredibly useful. But we don’t even know how to define it, except in terms of how we use it. In fact, the huge wikipedia article on energy doesn’t even have one sentence attempting to address this. The Department of Energy answers the question by telling you what energy can do, or what it allows us to do, but never tells you what it is. The Canadian government doesn’t do any better. In fact, this website gives the circular definitions:

A very good definition of energy is:
Energy is the ability to do work.
And a very good definition of work is:
Work is the transfer of energy.

All I can conclude from thinking about this is that energies, like velocities, don’t mean anything on their own; they only have meaning when we talk about it relative to something else. In the train on the left, if I ask the man in blue what his velocity is, he might say 75 miles per hour. Or he might say he’s at rest. It depends on whether he thinks I mean relative to the world outside the train or inside his train car.

Well, energy is only meaningful when we look at it relative to something else, too. If I give you a proton and ask you what its energy is, you can tell me how much electrical energy is in it and how much rest mass energy is in it. But you’re also telling me that’s relative to having no charges and no masses around. Does that mean that there is such a thing as “absolute zero energy” in the Universe? Well, there may be, but the best zero-point energy definition we have is not zero! Even completely empty space has energy in it; this seems to be the apparent dark energy that drives the accelerated expansion of the Universe.

Have any good thoughts on what energy is? Have a good definition of it? Leave a comment and share it with us!

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Q & A: The Age and Size of the Universe? http://startswithabang.com/?p=1739 http://startswithabang.com/?p=1739#comments Fri, 22 Feb 2008 01:30:40 +0000 ethan http://startswithabang.com/?p=1739 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. (It’s identified in the image at right.)

Want to get more accurate than that? There are other methods, too, like looking at radioactive element abundances (at left). 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

  1. how far away was it from us when the light was emitted
  2. how far away is it now that we observe it, or
  3. 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; see the illustration at right.) 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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