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The Economist has an article discussing the multiverse: Multiversal Truths. I’m mentioned in one of the figures. I’m not sure if Martin Rees would appreciate being called a string theorist, but otherwise the article is worth a read.

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Another talk in Sydney tonight. Everything you could possibly want to know about cosmology!

The Macarthur Astronomy Forum proudly presents Dr Luke Barnes (USYD)

When: Monday July 20th.

Where: UWS Campbelltown, University of Western Sydney – Bldg 30, Main Theatre – School of Medicine: Goldsmith Avenue, Campbelltown.

Time: 7.30pm sharp

Topic: Act One, Scene Two: How the Universe blooms.

How do we know what the Universe is made of? And what shapes its parts into the stars, galaxies and clusters of galaxies that we see around us? Starting from the very early universe, I’ll discuss how the fundamental factors of our universe, its forces, particles, and the dynamical stage that they tread (space time), compete and cooperate to fashion the Universe we see around us.

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Research question: Do telepathic powers exist? Such powers would be highly in demand, so highly in fact that telepaths might become paranoid and keep their abilities secret. Here, I propose a method to identify hidden telepaths. (more…)

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Another video of one of my talks. The goal is to take Bayesian probability theory as it is used in the physical sciences and see if it can make sense of postulating and testing a multiverse theory.

As part of a project called Establishing the Philosophy of Cosmology, I attended a conference in Tenerife, Spain in September last year. The line-up of fellow attendees was, frankly, intimidating. Nevertheless, I had a wonderful time, learned a lot and presented some of my own ideas towards the end of the conference.

The videos are now available on YouTube here; talk slides are here. Just about all the talks are worth a listen – I’ve been enjoying listening to them again. Here are a few highlights.

Joel Primack – Cosmological Structure Formation. A nice introduction to how the universe made its galaxies.

Barry Loewer – Metaphysics of Laws & Time in Cosmology. A very helpful talk on how to think about the laws of nature, and the place of probabilities therein.

George Ellis – Observability and Testability in cosmology and Cosmology: what are the Limits of Science. Made an important distinction between “big-C” Cosmology, whose purview is all of reality, and “little-c” cosmology, which is a branch of science about what physics and physical observations can say about the universe as a whole.

Sean Carroll – What Happens Inside the Wave Function? (I’ll let Sean explain here.)

The talks by Don Page, Bob Wald, Jim Hartle, Joe Silk, David Wallace, David Albert, Chris Smeenk, Brian Pitts, Tom Banks, and Jean-Philippe Uzan were very interesting, as were the discussion panels of Dean Zimmerman, Jennan Ismael & Tim Maudlin, and Janna Levin, Priya Natarajan, Claus Beisbart & Pedro Ferreira.

Here’s mine.  Enjoy.

(My sister is a TV journalist. I’m going to have to get some tips about not fidgeting, what to do with my hands, and not flubbing my words. I say “quantise” instead of “quantify” at one point. *cringe* My good wife has seen me give public lectures, and has commented that I appear to be on speed.)

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I’ve started a new project at the University of Sydney. I’m still at the same desk, but I’ll be doing something a bit different. More details soon, but basically I’ll be using cosmological simulations of galaxy formation to try to make precise the connection between the fundamental parameters of cosmology – like the density of matter, the lumpiness of the early universe and the cosmological constant – and the conditions required by stars, and hence anything that requires stars.

For a brief overview of why anyone would do this, he’s a short presentation I gave at the Australian Academy of Science’s “Australian Frontiers of Science – The edges of astronomy” meeting in December 2014. My talk starts at 25:09. I think it’s queued up below. The other talks are also well worth your time:

The edge of the Universe—a fundamental limit how much we can know? – Associate Professor Tamara Davis
The small-scale spatial limits to the Universe – Dr Alessandro Fedrizzi
The edges of knowledge —the ‘physics is done’ syndrome – Associate Professor Michael Murphy

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Gabriel Popkin has written a nice overview of some recent work on the fine-tuning of the universe for intelligent life at insidescience.org, titled “A More Finely Tuned Universe“. It’s well worth a read, and features a few quotes from yours truly.

It details the work of Ulf Meissner and colleagues on the dependence of the Hoyle resonance in Carbon on the masses of the up and down quarks. The quark masses are fundamental parameters of the standard model, meaning that we can measure them, but the model itself can’t predict them. They are just arbitrary constants, so far as the equations are concerned. Their work shows that a change in the quark masses of ~3 percent with respect to their values in this universe will not result in the universe producing substantially less carbon or oxygen, so this is something of a safe zone. As the article quotes me as saying, I hope that they continue to push things further, to see if and where the universe really starts to change.

I have a problem, however, with the following quote:

David Kaplan, a particle physicist at Johns Hopkins University in Baltimore, said two to three percent gives the quark mass a lot of wiggle room compared to other much more finely tuned parameters within physics, including the cosmological constant.

(Just to note: I was quoted accurately in the article, so probably the other scientists were too. This isn’t always the case in science journalism, so I’m responding here to the quote, not necessarily to the scientist.)

The three percent change in the quark masses is with respect to their values in this universe. This is a useful way to describe the carbon-based-life-permitting range, but gives a misleading impression of its size. For fine-tuning, we need to compare this range to the set of possible values of the quark masses. This set of possible values – before you ask again, Jeff Shallit – is with defined by the mathematical model. It is part of our ideas about how the universe works. If you’ve got a better idea, a natural, simple idea for why constants like the quark masses must have the values they do, then write it down, derive the constants, and collect your Nobel Prize. The standard model of particle physics has no idea why the constants take any value over their possible range, that is, the range in which the model is well-defined and we can calculate its predictions. Moreover, in testing our ideas in a Bayesian framework, we cannot cheat by arbitrarily confining our free parameters to the neighbourhood of their known value. The prior is broad. Fine-tuned free parameters make their theories improbable.

The smallest possible mass is zero; the photon, for example, is massless. The largest mass that a particle can have in the standard model is the Planck mass. Larger particles are predicted to become their own black hole, so we would need a quantum theory of gravity to describe them. Alas, we’re still working on that.

3% of the quark masses value in our universe is one part in $ latex10^{23}$ (one followed by 23 zeros) of the Planck mass. Technically, the down quark mass is (roughly) the product of the “Higgs vev” and a dimensionless parameter called the Yukawa parameter. The possible range of the Higgs vev extends to the Planck mass; why it is so much smaller than the Planck mass is known as the Hierarchy problem. The quark Yukawa parameters are about 3 \times 10^{-5}, which leads Leonard Susskind to comment (in The Cosmic Landscape),

.. the up- and down-quarks … are absurdly light. The fact that they are roughly twenty thousand times lighter than particles like the Z-boson and the W-boson is what needs an explanation. The Standard Model has not provided one.

In my paper on fine-tuning, I discuss the “cheap binoculars fallacy”: you can make anything look big, if you just zoom in enough. Actually, the fine-tuning of the cosmological constant is a good example of avoiding this fallacy. Relative to its value in our universe, the cosmological constant doesn’t seem very fine-tuned at all. Forget 3%; it can increase by a factor of ten, or take on similar but negative value, and the universe would still contain galaxies and stars. No one thinks that this is the answer to the cosmological constant problem, because comparing the life-permitting range with the value in our universe is irrelevant. When we compare to the range the constant could take in our models, we see fine-tuning on the order of one part in 10^{120}.

Later in the article, Kaplan states:

“Maybe if you change the quark masses not by three percent but by 50 percent you could end up with a situation where life as we know it couldn’t exist, but life as we don’t know it could exist,”

I agree with that sentence, so long as it starts with “Maybe”. But the state of understanding of our models is such that the burden of proof is now firmly on the “life as we don’t know it” claim. There is zero evidence for it, and piles against it. For example, one doesn’t have to change the quark masses by very much to obliterate nuclear binding. No nuclei. No atoms. No chemistry. No periodic table. No stars. No planets. Just hydrogen gas. These calculations have been done, for example “Constraints on the variability of quark masses from nuclear binding” by Damour and Donoghue. If they are wrong, then write a paper about it and send it to Physical Review D. Possibilities are cheap.

Of course, when Geraint Lewis and I publish our fine-tuning book, all this will be sorted out once and for all, bringing fame and fortune and a movie deal. Editing continues, so stay tuned.

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Apologies for the blogging drought. More soon. I couldn’t help but comment on something in the news recently.

Doing the rounds this week is a Wall Street Journal article by Eric Metaxas titled “Science Increasingly Makes the Case for God“. A few thoughts.

“Today there are more than 200 known parameters necessary for a planet to support life—every single one of which must be perfectly met, or the whole thing falls apart.”

I’m really hoping that his reference for the “200” parameters isn’t Hugh Ross, whom I’ve commented on before. The fine-tuning of the universe for intelligent life is about the fundamental parameters of the laws of nature as we know them, and there are only about 30 of those. Also, exactly zero fine-tuning cases require a parameter to be “perfectly” anything. There is always a non-zero (if sometimes very small) life-permitting window.

The fine-tuning for planets is a bit of a non-starter. How many planets are there in the universe? We don’t know, because we don’t know how large the universe is. There is no reason to believe that the size of the observable universe is any indication of the size of the whole universe.

Without a massive planet like Jupiter nearby, whose gravity will draw away asteroids, a thousand times as many would hit Earth’s surface.

This turns out to be a bit of a myth, however widely reported. Jonathan Horner and Barrie Jones used a set of simulations to test this idea, but their results tended to show that the opposite might be true. (more…)

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