Starts With A Bang! » inflation 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 How to Destroy the Entire Universe http://startswithabang.com/?p=1808 http://startswithabang.com/?p=1808#comments Wed, 23 Apr 2008 15:49:52 +0000 ethan http://startswithabang.com/?p=1808

Since the dawn of time man has yearned to destroy the sun. - C. M. Burns

There’s no need to stop at the Sun, though. Since yesterday was Earth day, I thought it was only appropriate to spend today telling you how not only to destroy the Earth, but to effectively destroy the entire Universe. To tell you this story, we have to go all the way back to the beginning, to just before the big bang.

The big bang was when the Universe was hot, dense, full of energy, and expanding very quickly. The Universe was also spatially flat and the same temperature everywhere, and full of both matter and antimatter. It may have looked something like this:

(Image credit: Stephen Van Vuuren, created from a simulation of 80,000 star images.) The thing is, we need something to make the Universe this way; we need something to set up the big bang. What makes the Universe flat? What forces the Universe to be the same temperature everywhere? What creates the fluctuations that allow stars, galaxies, and clusters to form from gravitational collapse? What pushes all the weird stuff that might have existed before the big bang away?

The best theory is cosmic inflation, or a theory that says that the Universe went through a period where space expanded exponentially fast. That expansion pushes everything that existed before away, removing it from what we know as our Universe. It takes whatever shape space is and stretches it flat. It takes a small, uniform area and stretches it, giving every point in our Universe the same temperature. And it takes tiny, quantum-scale fluctuations and stretches them across the Universe, creating those fluctuations that allow the formation of stars, galaxies, and clusters. It even gave the correct predictions for the amplitude and spectrum of those fluctuations, more than a decade before we were able to measure them!

So to destroy the Universe, all we have to do is make one tiny point near us expand exponentially fast again, even just for a tiny fraction of a second (~ 10-30 seconds), and that will remove everything we know of entirely, creating a new Universe in its wake. Kind of like a Phoenix (left).

Some of you may object. You may say that it’s wrong to do this; that this would be playing God. Look, people, if you want to destroy the Universe, there are some things you’re just going to have to suck up.

So how do we do this? In some sense, it’s as simple as pushing a ball up a hill; you just need enough energy. All we have to do is make the particle that causes inflation, called an inflaton, with enough energy to make the Universe inflate again. For instance, if we made an inflaton at the low energies we’re used to in big accelerators (you know, like 1012 Electron-Volts), we couldn’t get it out of the bottom of the valley that it’s stuck in, like this:

In fact, supermassive black holes produce cosmic rays that are about 1020 Electron-Volts, so we know we need more energy than that. But if we can get up to about 1026 Electron-Volts, we’re sure to do it. “Do it” means push that inflaton up the hill; push it up high enough, and you get inflation! And that’s how you destroy the Universe!

All you need is a bigger particle accelerator or stronger magnetic fields. We can get up to 1012 eV with a ring with 4 Tesla magnets and about a 1 km radius. So we’d really need a ring with a 1014 km radius (and the same magnetic field) to do this, or an accelerator ring about the radius of our distance to the nearest star. So support your particle physics research and the development of stronger magnetic fields, otherwise we’ll be doomed to celebrate many more Earth days!

Interesting note: Some of you were upset by my post that said string theory is untestable in principle. All you have to do is build a powerful enough accelerator. Guess what, the energy it takes to destroy the Universe like this is less than the energy it takes to test string theory (which is 1028 Electron-Volts). Have a nice day!

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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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Inflation’s Problems and Alternatives http://startswithabang.com/?p=1757 http://startswithabang.com/?p=1757#comments Fri, 14 Mar 2008 09:05:05 +0000 ethan http://startswithabang.com/?p=1757 This month’s issue of Physics Today has an interesting article by Robert Brandenberger of McGill University, entitled Alternatives to Cosmological Inflation. As a refresher, cosmological inflation is the theory that sets up the Big Bang: it takes whatever was in the Universe prior to inflation and expands it away, leaving you with a Universe that has roughly (to a few parts in 100,000) the same properties everywhere, and is spatially flat.

There are many models of inflation which give a Universe like ours, although we have to fine-tune the parameters of it. For instance, if we treat inflation like any other field theory, we need to specify the shape of its potential very precisely (and very much unlike real potentials we observe), we need to ensure that the tiny fluctuations are close to, but not exactly the same on all scales, and we need to ignore the fact that simple models of inflation still give you a singularity to start your Universe.

But the biggest problems with inflation are the fact that we don’t know how to make a realistic particle physics model of it. Another problem is that if inflation lasts a very long time (and almost all models of it do), then the scales that are the size of the Universe today were once really, really small. Smaller than an atom, smaller than a proton, smaller than the Planck length, and this is a problem.

Why? Because when you go smaller than the Planck scale, your laws of physics, like quantum mechanics and gravity, don’t make any sense anymore. We don’t know what the fluctuations should look like (see the image at right). So we have no idea what to make of the fact that there ought to be sub-Planck-scale physics signals littered throughout the Universe; this is known as the Trans-Planckian problem. With a view to addressing some of these concerns, Robert goes over three alternatives (and none of them are very appealing).

1. Add defects and vary the Speed of Light. This one’s out. Why? Because we would see defects in the Cosmic Microwave Background, and we don’t. The constancy of the speed of light is highly supported by experiments, but there is a theoretical disaster if it turns out that either c (the speed of light), G (the Gravitational constant), or h (Planck’s constant) changes: energy is no longer conserved in the Universe! It’s possible, but… yeesh!

2. Bouncing Cosmologies. These theories have the Universe go through cycles, where they have big bangs followed by expansions, turnarounds, and contractions, followed by a big crunch. There are models out there that have this happen in a manner consistent with what we observe, but they’re extremely difficult to reconcile with dark energy in the Universe, which we have. If these scenarios are correct, it means that this dark energy is a temporary, transient thing. Again, these models need to be very finely tuned to mimic the observations that have already been made.

3. String gas cosmology. At least it doesn’t have a singularity. Unfortunately, it also doesn’t really have the properties that our Universe does, like homogeneity; in other words, this scenario doesn’t solve the horizon problem. It also doesn’t explain why our Universe is so large, which is what inflation does.

Now to be fair, despite having a vested interest in string gas cosmology, Robert actually states the pros and cons of these various ideas, and the conclusion I draw from this, really, is to pick your poison. Nothing solves all the problems yet, but at least they are, in principle, distinguishable from one another through methods like measuring the spectral index (whether fluctuations are larger at small or large scales) of both matter and gravitational waves. Actually, PLANCK has a good chance of making some of these measurements, and it launches later this year! I’ll give you the update when we figure it out!

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