Archive for the 'Big Bang & Cosmology' category

"The End of Time" : somebody explain to me why this paper isn't all crap

One of my students this morning mentioned he'd seen a news article about scientists who suggested that time had a 50% chance of ending in the next 3.7 billion years, and that we have to accept this result if we accept the way we do our calculations.

The paper is arXiv:1009.4968, by Bousso et al. I have to admit that I am not in in the field, and so don't really know how people really do calculations for the probabilities of events in eternal inflation. But, as best I can tell, what this paper has done is taken a calculational tool that theorists use in order to make calculations tractable, and have interpreted it as reality.

Here's an analogy to what I think is going on. When you do "aperture photometry", you take a digital image of a star, and add up the number of counts in all the pixels that are less than a certain distance away from the center of that star. In reality, the probability distribution for photons from that star extend all the way out to the edge of the field, but it's not practical to try to add things up that way. So, if you care about the total light in the star (say because you're calculating your aperture correction, if you know what that means), you draw an aperture that's "big enough" that you know you're including 99.99% of the light from the star, and don't worry about the fact that you've thrown out the other 0.01% of the light of the star, as your other uncertainties and systematics are bigger than that anyway. The radius of your aperture is a cutoff you use in the calculation for purposes of making the calculation practical and tractable. It does not mean that you're interpreting your data to say that absolutely no light from the star can exist outside of that line. Yes, you're implicitly assuming that, but you don't really believe it. What's more, your calculation shouldn't be taken to mean that you require there to be no light outside your aperture for your measurement to have meaning, and to be useful.

As best I can tell, Bousso et al. are taking a similar cutoff that theorists use when doing calculations in an inflationary universe, and are interpreting that calculational cutoff as The End Of Time (dramatic chord).

Can somebody explain to me why this isn't just overinterpretation of an approximation made for the sake of finding numbers? Why this is any different from saying that "if you use Kepler's Laws to calculate the orbit of Mars, you will float away because the validity of your calculations depend on Earth having zero mass"?

2 responses so far

Why Our Universe is Almost Certainly Somebody's Simulation

Despite the title, this blog post is really here as a warning to others about trying to reason from first principles about the nature of our Universe...

We have already done sophisticated simulations ourselves that do the large scale simulation of structure in our Universe. Yes, this is for a small fraction of the total Universe, and, yes, it only simulates down to galaxy size. (That is, it doesn't simulate small enough areas to model the formation of stars or anything like that.) But the point remains that we're simulating the Universe.

We've also done "artificial life" simulations, in which we've shown that computer programs evolve. One of the most interesting physics colloquia I heard as a grad student at Caltech in the first half of the 1990's was about the Tierra project, that created an "environment" in which small computer programs competed for resources. It had a mechanism for random mutation, and over time the programs evolved to become more efficient.

Take these simulations, and allow for computer power to continue to improve as we've seen it improving in recent decades, and it really doesn't seem too much of a stretch to imagine that we could create a simulation of the local area of a galaxy that has the computer power necessary to evolve very complex "organisms"-- simulated organisms, that is. Code in the basic laws of physics, and give them enough space and computer power to evolve, and it could just happen.

Now, consider our Universe as we've observed it to be. We only know of one life-bearing planet, but we do know that there are lots of other planets in our Galaxy. And, looking out there, within the observable Universe (not even considering things so far away that light hasn't had time to reach us since the Big Bang), there are something like 100 billion galaxies like ours. Given how tenacious life is on this planet once it got started, even if it's rare for it to get started (say even only one or a few instances in our own Galaxy), there are certainly other planets out there with life on them.

If we use our statistical sample size of one, so far we see that a few hundred years out of a few billion years of evolution includes a technological civilization. That's a small fraction... but given the number of stars in our Galaxy, and the number of galaxies in the observable Universe, it means that there are other technological civilizations out there, somewhere. Let's assume that an appreciable fraction (i.e. anything more than an infinitesimal fraction) of these civilizations eventually are able to produce computation able to make the kinds of simulations I'm talking about above. It's been for the last 7 billion years or so that it's reasonable to suppose that life could arise on planets like our own. During those 7 billion years, there have almost certainly been lots of these simulations run. (What does "lots" mean? Well, the numbers going into this are very uncertain, of course, but it's probably somewhere between hundreds and billions.)

In other words: for one observable Universe like our own, in which intelligent creatures could arise, there are many simulations in which intelligent creatures could arise. Thus, any given civilization of intelligent creatures is by far more likely to be within one of the simulations rather than in the real Universe.

So, we're probably all part of somebody's simulation.

Right. Do I really believe that? No. I mean, maybe, but if so, so what?. If the simulation were done well enough, though, we'd have no way to tell the difference. So, at some level, trying to decide if we're somebody's civilization or if we're in a real Universe is mental masturbation. We see a Universe out there which has physical laws that are Universally obeyed. The process of science has a great track record in explaining how this Universe works and predicting what we will observe. Hence, it makes the most sense to go with the simplest explanation, that there is a real Universe and we are working on understanding it. If certain things about our Universe seem improbable from first principles-- for instance, why are the densities of Dark Matter and Dark Energy so close?-- it's worth thinking about whether that's a pointer to something deeper. But, at some level, the Universe is what it is, and it's worth trying to understand it without getting hung up on fundamental probabilistic arguments that lead us to thinking that none of it means anything anyway.

13 responses so far

The History of the Universe

You're going to read a lot about science on Scientopia. This post is to help you put it all in context. I'm not going to go into great depth, because that would take, well, the effort of humanity over many lifetimes. This is just a drive-by overview.

The slide below is one that I show in a substantial fraction of the talks that I give:

A History of the Universe

The first thing to notice about it is that it's a logarithmic scale. In the top line, at each tic-mark the universe is about 10 billion times older than it was a the previous tic-mark. That sounds like a lot, until you look at the labels on the tic-marks... the last mark on the top line is 109 seconds, or about 30 years.

Since the Universe is only about 14 billion years old, another factor of 10 billion past that step would take the Universe past the present day. Thus, the black circled region on the right of the top line is expanded into the bottom line, where each step is only typically a factor of 30 in age. This isn't exactly right, because really it's a step of a factor of 10 of redshift. That's what the variable "z" is in the figure, and it's very important, but I'm going to have to save it for another post. Likewise, the variable "T" along the timeline is the ambient temperature of the Universe, which also requires a lot of additional exposition, and so I will put that off to a future post. Continue Reading »

9 responses so far

Answering Objections to the Big Bang

Every so often you will come across somebody who has a "killer" list
of "problems" with the Big Bang. While there remain unknowns and
questions about the Big Bang— just as there do with biological
evolution— the basic picture of the Big Bang is rock
solid— just like evolution.

Nearly two months ago, I received a query from somebody who found my
name through the
Clergy
Letter Project "expert database"
regarding one of the websites that
lists these objects. I've been through quite a number of life changes
in the last 6-8 weeks, and my blogging rate has suffered as a result.
However, I'm finally getting to it. Nearly all of the things I will
respond to here are generic responses, as these "objections" to the
Big Bang are frequently brought up, but for reference I will link to
the site that was given to
me: Dr. Tom
van Flandern's Top 30 Problems with the Big Bang
. Nearly all of
these objects are either a misunderstanding of the Big Bang, or
an objection that is out of date. I won't address all 30
individually, but I will hit some of the highlights. The fact
that I don't address a given objection should not be taken as
evidence that I'm ceding the point!

Continue Reading »

8 responses so far

The Gruber Prize Ceremony

Hello from England! Last night was the award ceremony for the 2007 Gruber Prizer in Cosmology. It was good to meet up with the members of the SCP again, many of whom I haven't seen in several years. Picture below is the members of the collaboration who were present, underneath one of the many trees labelled as "Newton's Apple Tree" in the UK; this one is at Trinity college, and apparently is a descendant of the original tree from the apocryphal story about the apple falling on Newton's head. Here we are all looking for apples of our own, but evidently these apples are made of dark energy and as such are not falling into gravitational potential wells....

P1030415.JPG

At the awards ceremony, a number of people spoke. I want to comment on three things that people said. First, Jim Peebles, one of the previous winners of this award, thanked the awardees for two things. First, for solving one problem: the mass density of the Universe, and fixing the cosmological "age crisis" through the measurement of a positive cosmological constant. Second, for the introduction of a new conundrum: just what is this cosmological constant or dark energy stuff? There's nothing scientists like better than a good conundrum.

The two individual winners of the award, Saul Perlmutter and Brian Schmidt, each spoke, and each had similar themes to what they said. Saul started by noting that there is this popular image of the lone scientist working on brilliant discoveries all himself, but often it doesn't work that way. He then went through a serious of snapshot images he has in his head of the process of discovering the acceleration of the Universe, mentioning the names of each of the rest of his team who were present. (About me, he said that I type and program faster than he talks... and if you've heard Saul talk, that's saying something!)

Brian Schmidt, likewise, came up and said that he wasn't speaking for him, but he was speaking as the representative of the High-Z team. He said that a lot of the problems that are present in science today can not besolved be individuals, despite the fact that many prizes are still given to individuals. Rather, they are solved by teams.

It is significant to me that half of this award went to not individuals, but to the teams of which Saul and Brian were leaders. There were many, many contributions all worthy of recognition.

If I were more cynical I might say that Saul and Brian were just saying what they were supposed to say, facing the fact that about 1/3 of the people in the audience were the other team members.... However, I am not that cynical. I really believe that what Saul and Brian said was heartfelt. I appreciate both of them and the Gruber Foundation for so publically recognizing that this discovery did require teams, and that those teams were worth of honor.

I close with just one picture from the dinner later last evening. Alex Kim, who is pictured, was a graduate student at UC Berkeley when I and Peter Nugent arrived as post-docs in 1996. Ironically, Alex Kim defended his thesis in late 1996, but both Peter and I defended our theses in early spring 1997... for a while, the post-docs didn't exactly yet have their PhDs, but the grad student did! You can see that Alex (now a permanent staff member at LBNL, after a stint as a post-doc in Paris) has done well, for he seems to have located a new standard candle....

P1030434.JPG

Photograph by Nelson Nunes

3 responses so far

Second Life Q&A on the Accelerating Universe

Following the talk I gave in Second Life about the discovery of the accelerating Universe, we held a couple of Q&A sessions. The original plan was to have questions right after the talk, but the Second Life main grid crashed right at that moment. We all got online about half an hour later, and I held one Q&A session for the people who came back. There was another one the next day.

Troy McLuhan (his Second Life avatar name) logged the session, and has done the hard work of formatting and lightly editing it for web publication. You can find the transcript of the Q&A session online here.

2 responses so far

Accelerating Universe Talk Transcript & Followup

I managed to get through my 15-20 minute "talk," and just as I threw it open for questions Second Life had a database problem and everbody in-world had to be logged out.... We got back in 40 minutes or so later, and I answered questions for a while for people who came back. However, if you were at the talk and wanted to ask questions but didn't come back, I'll be doing a follow-up Q&A session tomorrow (Wednesday August 1) at 10AM PDT at the same location.

Below, I've got a transcript of the talk I gave. Other than fixing some typos and merging things into paragraphs, I haven't edited what I said/typed.

Continue Reading »

4 responses so far

Accelerating Universe Talk in Second Life Today

Jul 31 2007 Published by under Big Bang & Cosmology, Second Life

Just a reminder: I'm giving a talk / Q&A session about the discovery of the accelerating Universe today in Second Life. The talk is at 10:00 AM PDT / 12:00 Noon CDT / 1:00 PM EDT / 17:00 UT. Find it by going to this location: Spaceport Bravo (120,65,278).

Also, for those of you who don't know: a basic Second Life account is completely free! Go to the site and register for an account, and download the client to run on your computer. After you get in-world, you'll go through an "orientation island" that teaches you how to move about and look about. The Basic Account lets you get in world and go anywhere in world. You can build things if you find a "sandbox" (which are plentiful). You don't have any money to start, but there are all sorts of freebies out there if you want to customize your clothing and appearance, or if you want random toys to play with. Many people exist with just a basic account.

(A premium account lets you own land-- on which you can build things. You also get a small stipend of "Lindens" (the in-world currency) per week. With just a basic account, if you want to buy things in world, you can buy Lindens with a credit card. There are more details of course (specifically with regard to land; there *are* ways to have land with just a basic account).

2 responses so far

Accelerating Universe Talk in Second Life

Next Tuesday, July 31, at 10:00AM PDT (17:00 UT), I'll be giving a talk and Q&A session in Second Life about the discovery of the accelerating Universe.

The talk is being hosted by Troy McLuhan of the Science Center Group. I t will be located at Spaceport Bravo (120,65,278).

And now, for no adequately explained reason, I include a picture of my Second Life self wearing M51 on my head:

prospero_galhat_400x317.jpg

8 responses so far

The Gruber Prize in Cosmology is awarded to, among many others, me

This is really cool. Several years ago, the Gruber Foundation established a prize in cosmology. Last year (2006) the award went to John Mather and the COBE team; you may recall that Mather was one of the two winners of this year's Nobel Prize in Physics. This year the award is being split four ways: (1) Saul Perlmutter, leader of the Supernova Cosmology Project (SCP); (2) Brian Schmidt, leader of the High-Z Supernova Team (HZT); (3) the members of the SCP who were on the Perlmutter '99 paper; and (4) the members of the HZT who were on the Riess '98 paper. These two papers were the refereed-journal announcements to the world of the discovery that the expansion of the Universe is accelerating. (Both teams had previously made announcements at conferences, starting with the January 1998 AAS meeting.) Anything you hear about Dark Energy today comes, to some degree, from that discovery. Although the term doesn't predate that discovery, scientific thinking about it does, and indeed goes all the way back to Einstein. However, the tremendous interest in it today comes from the observations of the accelerating Universe which told us that Dark Energy (a more general form of what is called the Cosmological Constant) is, probably, real.

What to me is coolest about this award is that it's going to the groups. Usually in science we honor and award the Warrior Hero, the single Big Name who was the brilliant and creative scientist who did everything. The Nobel Prize went to Smoot and Mather. Assuredly the reason for the Nobel Prize was extremely worthy, and assuredly those two gentlemen deserved it. But the Gruber prize recognized the team without which Mather could never have done the work that he did in order to win the Nobel Prize.

I will speak of the SCP, because that's where I was. HZT people, please don't feel neglected, but obviously I don't know the internals of that nearly as well.

As for the division, I like it. I think Saul Perlmutter really does deserve to split it halfway with "the rest of the authors of the paper." Saul was unambiguously the leader of that effort. It was Saul's vision, drive, tenacity, and confidence in the face of huge obstacles that let the high-redshift supernova search succeed in finding the kind of supernovae needed to make the measurement we needed to make. (It was luck, of course, not Saul's doing, that the measurement came out with such an amazingly cool and, to many, unexpected result.) But, of course, many of the rest of us devoted a lot of creative energy and talent into making this project work. I did my post doc from 1996-2001 with the Supernova Cosmology Project, the most exciting result of which was the Perlmutter '99 paper. I kept working with the SCP on this stuff for a few years after I got to Vanderbilt, culminating in the Knop 2003 paper. I, along with many others listed in the names of the "Supernova Cosmology Project," will never be a remembered name in the annals of the Warrior Hero Scientists, but it is nice to see the lot of us receive some real recognition as a group for the group effort that went into this remarkable discovery.

It's a $500,000 award, but given that I'm sharing a quarter of that award with about 30 other people, my payout will be a couple of orders of magnitude less. I may just be able to pay off a student loan that I've been paying down for a deade or so now....

The formal award will be on September 7 at the University of Cambridge. At the moment, I don't know if I will be able to attend. Not only is that expensive to get to, but it looks like it might directly conflict with travel I will need to be doing for the new job I'm going to be starting.

In the near future, I will try to write my "definitive" blog posting in which I describe, hopefully for a general audience, how the supernova observations tell us that the Universe is accelerating. I"ve given a few different talks about this to a popular audience, most recently at a few Shapley lectures this year, and in June 2006 at Hypericon.

19 responses so far

« Newer posts Older posts »