Current Cosmology From Supernova Data.

Ariel Goobar and Bruno Leibundgu have recently submitted an article to Annual Review of Nuclear and Particle Science summing up our current understanding of physics from the current set of supernova data. We have accrued quite a lot of supernova data over the years and so it is interesting to take a look at how much we have learned. I will not report everything but will post a few interesting plots.

Above is the original diagram/scatter plot Hubble used to show the universe is expanding in a way that fits Hubble's law. This is that same diagram today using current supernova data (not a scatter plot any more!): (showing the distance modulous versus redshift.)

As you can see Hubble's law is confirmed by quite a few supernova today. :) Furthermore, the lower plot shows a blue line representing a universe containing cold dark matter and a cosmological constant and a flat dotted line assuming a universe empty of cold dark matter or dark energy/cosmological constant. As can be seen, the supernova data *strongly* favors a universe with dark matter and dark energy/cosmological constant.

The next plot above shows how well we can constrain the percentage of dark matter and dark energy in the universe using supernova, CMB and BAO data. (click on the image to see better.) As you can see the data fits a flat universe with an accelerated expansion very well

The last plot I want to display shows the current constrains we have on the type of beast dark energy is. As a reminder, the prediction we get from dark energy being the cosmological constant is w = -1. As you can see w = -1 still fits the data very well.

Conclusion: It is nice to see as more and more cosmological data pours in the standard flat universe containing dark energy, cold dark matter, accelerated expansion and dark energy best described by a cosmological constant is verified. Cosmology has truly become a precision science.

Ariel Goobar, & Bruno Leibundgut (2011). Supernova cosmology: legacy and future To Appear In Annual Review of Nuclear and Particle Science arXiv: 1102.1431v1

Dark Matter Reconstruction From Radio Experiments.

As photons move through the universe they get gravitationally lensed as the pass by large clumps of matter. (As shown in the image above.) Dark matter, being the dominant form of matter, lenses these photons more than anything.  Therefore, by studying the lensing properties of incoming photons, in principle we can reconstruct what the profiles of the dark matter doing that lensing.

Now, put (hopefully) more simply: we can't directly see dark matter.  But taking the statistical lensing properties of the incoming photons we can hopefully reconstruct what the dark matter looks like.  In this way we can "see" the clumps of dark matter in the universe directly.

Recently, Brown and Battye have proposed a new method for reconstructing the projected dark matter distribution using radio surveys like SKA and e-MERLIN. They then test their method on simulated data as discussed below.

The plot above shows a mock initial dark matter distribution from which they make simulations of the kind of data e-MERLIN would see coming from such a distribution of dark matter.  If their method works, this is the distribution they will reconstruct.

This next plot above shows how well they are able to reconstruct the input dark matter distribution.  As can be seen, the main features, especially the two dominant clusters, can be resolved fairly well.  (Especially given the input data is smoothed by the beam of the instrument and so this at some level is approaching as good as things get for a single experiment.)

Punchline: So, using this lensing reconstruction technique for be can begin to "see" what the underlying dark matter clumps look like.

I for one am excited about where lensing is headed.  Utilizing a variety of lensing techniques, including the method for radio sources done here, we may one day reconstruct the dark matter throughout the universe with great precision, especially when we combine the data from many experiments at many wavelengths.  Again, in this way we can visually "see" the dark matter that comprises our universe.


Michael L. Brown, & Richard A. Battye (2011). Mapping the dark matter with polarized radio surveys E-Print arXiv: 1101.5157v1

What If Dark Energy Were A Phantom Energy?

Before we get too far ahead of ourselves, let's remember that dark energy being a cosmological constant fits the data very well and has for years. That said, experimental constraints allow for dark energy actually being an exotic form of phantom energy. (So for the time being we have to allow for the possibility and work out the details.) This was recently done by Dabrowski and Denkiewicz.

What Is Phantom Energy?  Normal matter/energy in cosmology is classified according to the equation of state:

where p is the pressure and ρ the energy density of the matter/energy.  For radiation w = 1/3, for matter/dust w = 0 and for the cosmological constant w = -1.  What's interesting to note, looking at the image at the top, is the larger (more positive) w is, the faster it dilutes in the universe as the universe expends.  The cosmological constant is right at the point where it's density remains constant throughout the expansion of the universe.

Phantom energy is energy that has w less than -1.  If this form of energy existed, it would actually increase in density as the universe expanded!

Does It Fit The Data?  The best constraints on w for what is driving the dark energy is w = -1.05 +/- 0.29 from supernovae, CMB and 2dFGRS data and w = -1.001 +/- 0.0129 which hardly rules out dark energy actually being a form of phantom energy.

Furthermore, as the above plot shows, certain phantom energy models do fit current data, such as the supernova observations shown in this plot, and so we have to be willing to probe these models especially if they further go on to make experimental predictions. But do they?

Possible Experimental Prediction: A Sudden Big Rip.  First, as stated above, phantom energy models by definition predict w to be less than -1.  This itself can be measured and therefore tested.  Second, if we can somehow demonstrate that the energy density of the universe is is increasing with expansion, that would be a tale tale sign.

But lastly I want to discuss the big rip. It turns out, that if dark energy is driven by phantom energy, the universe gets ripped apart in finite time. More technically, the scale factor "a" controlling the expansion of the universe becomes infinite in finite time.  This means the distance between you and everything else in the universe goes to infinity without having to wait an infinite amount of time for this to happen.

In fact, a model discussed in this paper predicts the universe will experience this big rip in only 8.7 million years from now!

No wonder the authors call this model the "sudden future singularity" model. If the universe becomes singular, the scale factor "a" becoming infinite in only 8.7 million years, compared to the age of the current universe that will be one sudden singularity!

Conclusion.  I do want to remind people that the standard cosmology scenario where dark energy is a cosmological constant has worked so well for so many years that we have no reason to abandon it.  That said, dark energy being driven by phantom energy is technically a possibility and so we are justified looking into it, especially since it's existence may be experimentally verified/falsified.

Basically if we ascertain w is less than -1, see the energy density of dark energy increase with expansion, or find our universe suddenly being ripped apart we will know the dark energy is actually a form of phantom energy.

Let's hope that last one doesn't happen any time soon. :)

Mariusz P. Dabrowski, & Tomasz Denkiewicz (2009). Exotic-singularity-driven dark energy AIP Conference Proceedings, 1241 arXiv: 0910.0023v1

Evidence Against The Universe Being Fine Tuned For Life.

Many people will tell you that the universe appears fine tuned for life.  Don Page has decided to address this issue scientifically by calculating the best value for the cosmological constant needed to support life in the universe and then comparing it to our own.  His conclusion is that the cosmological constant is actually an example that our universe is not fine tuned for life.

The cosmological constant is like a knob that affects how quickly the universe's expansion is accelerating or decelerating.  As a rule of thumb, the more positive the constant is the faster the expansion accelerates and the more negative the more it decelerates.  If it is zero, and there is just the right amount of matter, the universe just stays flat and we never experience a rapid acceleration or deceleration in expansion.

First, all positive values are bad.  Now, what does this have to do with life?  It turns out a positive cosmological constant, like our own, actually dilutes matter and prevents a lot of gravitational collapse making our universe less likely for life than if the constant were not positive.  From the paper:

The reason is that a positive cosmological constant gives a repulsion between separate particles that reduce the ordinary gravitational attraction and leads to less gravitational condensation of matter. Therefore, other factors being equal, any positive cosmological constant decreases the fraction of baryons that condense to form galaxies and other structures that eventually form living substructures.

As an immediate consequence, no positive value of the cosmological constant (such as the observed value Λ) can maximize the fraction of baryons in life 

But wouldn't a negative value also be bad? Yes, because if the value is too negative the universe recollapses and life doesn't have time to form.  Page keeps this in mind while calculating the best value to find that Goldilocks region that is most optimal for life.  That said, he does find that the optimal values for life in the universe are slightly negative on the order of Λ ~ -10^-120.

So God created a Multiverse?  Interestingly enough Page is very religious and so does not conclude this is evidence against God but actually evidence that God must have created a multiverse where each pocket universe has a different cosmological constant like most modern cosmology theories predict.  From the paper:

It might be appropriate to note that although this paper has focused on the scientifically testable question of whether the constants of physics maximize a particular measure for life, it obviously also has theological implications. It could be taken as negative evidence for theists who expect God to fine tune the constants of physics optimally for life. However, for other theists, such as myself, it may simply support the hypothesis that God might prefer a multiverse as the most elegant way to create life and the other purposes He has for His Creation.

I for one am a big fan of the multiverse because all modern cosmological theories with inflation lead to a multiverse.

And religion aside, given our cosmological constant is such a bizarre value and currently seems to be best explained by multiverse models I will agree with Page that the cosmological constant seems to hint at a multiverse. (Which is why many respected theoretical physicists suggest the peculiar value for the cosmological constant is the best evidence so far for crazy multiverse models like the string landscape.)

Thoughts?

Don N. Page (2011). Evidence Against Fine Tuning for Life E-Print arXiv: 1101.2444v1

How Physics Changes With F(R) Gravity.

Einstein's general relativity rules the roost when it comes to gravity, but soon modifications to standard GR may detected on universal scales with new cosmological data. Fortunately, a realistic modified version of gravity, known as f(R) gravity, makes practical predictions that may be verified in the coming decades.

A recent paper by Motohashi, Starobinsky, and Yokoyama gives a good synopsis of the general predictions coming from f(R) gravity. But first, what is f(R) gravity? Well, (and if this sentence is too technical just ignore it and read the rest) it is a gravity defined by the following action:

This is identical to the Lagrangian used in Einstein's general relativity except for the addition of the second term on the second line. Therefore, normal GR is equivalent to f(R) gravity for the case when λ = 0.

New Predictions: With the definition out of the way let's get started.

The gravitational constant G: The plot above is of the gravitational constant coming from f(R) gravity, called G_eff, divided by the same constant in GR (and Newton) evolving over time.(The variable z is known as the redshift and is a good measure of time for cosmologists and astrophysicists because it is logarithmic in nature and corresponds to the observed redshift of light.  See link for more details.)

What is important to note is that, unlike G for Newton and GR, the gravitational constant in f(R) gravity increases over time and evolves differently on different length scales! Thus, if the gravitational constant increases with time and evolves most significantly on the largest scales it could be a hint at f(R) gravity.

The dark energy parameter w:  If dark energy is the cosmological constant, than it is characterized by w=-1 in the equation that related the energy density to the pressure of the universe. (Energy density = -1 * Pressure)  And, all experiments so far confirm w = -1 so the case for the cosmological constant being dark energy is strong.

However, what could we say about dark energy if we find w does not always equal -1?  It turns out this is the case in f(R) gravity and even more peculiar is it crosses from being greater than -1 to less than -1 which throughout the history of the universe which is a fairly robust feature.  If this crossing could be observed in the future it would be a big win for f(R) gravity and could not be explained by the dark energy being the cosmological constant.

Okay, any tests on the theory today? Right now there aren't many because general relativity works so well it would only fail on the most extreme scales.  (Which we have a hard time probing.)  But the authors do include one interesting observation.

On the left is of the matter power spectrum assuming neutrinos are massless.  The green line is for standard GR and the others are different f(R) models.  The red error bars come from SDSS and show standard cosmology with GR is a good fit.  Now, the plot on the right shows the matter power spectrum assuming neutrinos are massive.  So if neutrinos are heavy, like for the black line, then f(R) gravity actually fits the data better than GR alone.

Conclusion:  f(R) gravity is not the only version of modified gravity that exists but it certainly makes practical predictions.  Also, not wanting to make this post 20 pages long, I did not report all predictions the theory makes.  However, these are the basic predictions for familiar physics provided by Motohashi et al. and I for one can't wait to see these f(R) theories tested in the coming years.

However, even if f(R) theories are verified, by examining the action above we can still conclude Einstein got the first and by far most dominant term in the Lagrangian right on the money!

Hayato Motohashi, Alexei A. Starobinsky, & Jun'ichi Yokoyama (2011). f(R) Gravity and its Cosmological Implications to be published. arXiv: 1101.0716v1

Thoughts On New Atheism. (Yet More Pet Peeves.)

Time for another post for my pet peeves series. I would like to address this new atheism that seems to be promoted by more and more.

I will not be blogging much about this as I think promoting the literacy and the love for science is helpful, whereas the science vs. religion debate is fruitless. Those who spend all their time engaging it completely have their priorities mixed up. (Like any extremist-minded person does.) People: spend some time promoting good science and not always attacking things science says nothing about!

Seriously!

1. Firstly, I Don't Take Issue With Them Being Atheist.

Let me start out with saying I do not take offense that they are atheist. I actually understand where many are coming from. They see that many traditional claims made by religious people seem untrue (like the world being only 7000 years old) and see many religious people rejecting basic science like evolution. Furthermore, they think a world with no God fits the data better than one that does. They just don't see evidence for God in nature so they have a hard time believing God is there.

Fine. I'm not going to discuss this here. In am not attacking atheists in general but these new atheists who not only oppose religion but for whatever reason seem obsessed with it.

*Also, I apologize in advance if I appear to be stereotyping all atheists.* They are not all like this... but some are.

2. They're Being Hypocritical When They Make Claims Without Applying Basic Scientific Rigor.

A quote:

They trot out tired, half-truthful stereotypes, and they cherry-pick historical examples of religious wrongdoing while ignoring the innumerable instances in which the faithful have performed great acts of decency and charity.

I will make this simple for them to understand: They can't get their claims published in established, reputable, peer reviewed journals.

They will claim how important it is to use evidence, good intellectual rigor and tout how believable claims should be strong enough to stand the peer review process. And yet their new atheist claims usually are not! (And they hope we accept their unpublished claims!)

Where has Dawkins published a paper in a reputable journal demonstrating religion is more harmful than good? (As he claims.) Where are the articles showing religious people fall into the stereotypes they glean from cherry picking history? (As any good scientist knows not to do.) They reality is, when it comes to scientific rigor, new atheist are hypocrites. (Or show me their own peer reviewed journal articles backing such claims!)

Seriously!

3. Many Have Become Fundamentalists Themselves. (Often Belittling Scientists Who Are Better Scientists Then They Could Ever Dream Of.)

Some new atheists have become as fundamentalist minded as the very people they deride. They live in a world where everything is black and white with no grey. I personally know of many who can't stand someone is a great scientist... if they happen to even be understanding of religious people.

Even if I won a Nobel Prize for physics (which won't happen by the way) all they would see is the fact that I am LDS and wrote a post like this. For example, take the Templeton Prize winners. Many of these people are better scientists than the vast majority of all these new atheists will ever be and yet they are often derided and treated with ridicule.

Once you have crossed the line to demeaning scientists who are 10 times the scientist you will ever be because they find meaning in something beyond the realm of science... you have officially become an extremist.

Seriously!

4. Many Hide Behind Science To Promote Their Pet Agendas.

New atheists are no different than than politicians who use the the politicization of science to promote their pet agenda! The fact is: science neither confirms nor denies the existence of God and these people try to hide behind pseudo-scientific arguments to do just that. Anyone who tries to to use science to prove God does or does not exist is wasting everybody's time and is being just as disingenuous as the above politicians.

Science cannot prove the existence of God, the Easter Bunny or for that matter the existence of good, love, beauty, morality, etc... And yet they are so quick to write volumes on how foolish people are for finding meaning in the former entities and yet not the later ones. There is definitely a biased agenda at play here.

Seriously!

5. What They Should Do.

If you are an atheist who is concerned with the horrible level of science literacy in the world, fine. I'm not attacking that. What I am attacking is a hypocritical, non-rigorous, fundamentalist agenda some have taken up pretending it is somehow backed by good science!

What these people need to do is get their priorities in order:

1. Demonstrate the wonders and benefits from accepting good science.
2. Demonstrate how much progress has been achieved from humans using science at their disposal.
3. Stick to claims that can be backed by the peer reviewed literature and avoid pet agendas that have nothing to do with science.

Edit:

Since I wrote the above I wanted to add this for reference:

6.  Atheism Causes You To Be Unproductive And Unsuccessful In Life.

Look at the above two images.  This study clearly shows that while atheists have a higher IQ on average, they also end up with less money than other people on average.  Obviously atheism causes you to be unproductive, unsuccessful and live largely below your potential.  How many politicians, generals, famous actors and actresses or successful businessmen are atheist?  Basically if you are smart but don't want to amount to much in life, studies that you should take up atheism.

Obviously this is a joke, but one made to prove a point.  There are probably 100 reasons besides atheism to explain why atheists have above average IQs and yet cannot figure out how to make as much money as the average person or hold political office, etc...  I recognize this.  It would be dumb of me to jump to such conclusions from such a non-rigorous interpretation of data such as.

I just wish some atheists would give religious people the same courtesy.   I wish they would pause and ask themselves if they have sufficiently gone through the 100 reasons, other than religion, to explain the data they like to flaunt?  For a people with such high IQs, I must say it seems like such an intellectually stupid thing to do.

Please just stick to promoting good science!  Blanket lumping together and attacking religion in general is often done unscientifically, is unproductive and a waste of time. (It is as unproductive and intellectually disingenuous as me writing books on how atheists are just unsuccessful eggheads.) But then perhaps people prone to atheism are naturally good at being inefficient and unproductive and so I just have to realize that trying to tell such people to do something helpful with their lives won't work. Again, joke. :)

snArXiv.org, And Fun With Abstracts For Theoretical Physics

If you've ever seen a title and abstract for a theoretical physics paper, you are going to love this.  It's called snArXiv.org, and hosts titles and abstracts for fake theory papers.  The above title and abstract is one example.  It's completely fake and randomly generated by the site's author David Simmons-Duffin using trends from the real ArXiv.org.

I must say it is pretty clever.  Honestly, many of these titles and abstracts look pretty similar to what is actually posted on the real physics pre-print site. (Though obviously wouldn't pass peer review.)  It just goes to show how crazy the theory literature can be.   This article is even co-authored by Feynman, Heisenberg and Higgs! Obviously the fake ones of course. :)

Just How Important Is WMAP?

We had some prospective grad students visiting Irvine the other day.  I said something about WMAP and one of the prospectives said "What's WMAP?"  Coming from a lay man I can understand, but a physics major!  I was in shock.  Here's why:

First off, "Since 2000, the three most highly cited papers in all of physics and astronomy are WMAP scientific papers." (Emphasis added.)

Second, today WMAP is just a relevant as ever.  I draw your attention to the most cited papers in 2009.  (The first one, Review of Particle Physics, isn't a research article but an "encyclopedia" people quote for values of things like constants. We've discussed this book before.)

In 2009, the #1, #3, #9 and #10 most cited research articles were the WMAP papers.  To be in physics and to not know about WMAP, to me, means you are living under a rock.  There is no experiment in this last decade producing more follow up scientific research!

I point you to two other interesting observations from that list:

First, the #4, #5 and #6 most cited papers from 2009 are other cosmology papers.  That means 7 of the top 10 most cited papers, tracked by Spires, are cosmology papers.  Cosmology is hot baby!

Second, the #2 most cited paper last year is the famous paper by Juan Maldacena introducing the ADS/CFT correspondance.  As pointed out by Peter Woit, at this rate this paper will surpass "Weinberg’s 1967 paper as the most heavily cited particle physics paper of all time."

Do Scientists Sometimes Publish Just To Be Cited?

Speaking of the Hořava gravity excitement, Luboš Motlhad this to say about four months after Hořava's initial publication:

Fifty papers have been written about the Hořava-Lifshitz gravity (NYU about it). Aside from the first author - Petr Hořava - and the most recent group of authors, everyone in this list seems to have gotten carried away...

They knew that someone would refer to them, whatever they write, so they often (incorrectly) connected the new bandwagon to their older work and/or offered solutions that would only be interesting if the theory actually worked...

So Motl seems to be implying that, outside of a few important papers like the one from Hořava, some scientists published papers largely to catch a bandwagon wave that was sure to bring lots of citations, even if the work was not high quality.

I'm not going to speculate whether or not this is true.  However, it raises an interesting question: Do scientists sometimes publish papers because it is a good opportunity to generate citations, even if the quality of the paper is not that great?

I guess even scientists are human.  That said, I'm not sure it is the ethical thing to do.

The Principle of Peer Review

The recent posts by Joe discussing some of the major ideas of science got me thinking, so I decided to wade in on the conversation and add a few things that Joe may not mention. I apologize if this is a little long, but I was trained as a philosopher so I may tend to be wordy.

For many people their only connection or exposure to science comes from a high school science class (or college), or from science centered magazines such as Scientific American or Popular Science. For some people their only exposure to science and scientists in general comes from Hollywood or TV (shudder). For all those who don't have personal experience with scientific research there are a few critical things that never seem to get mentioned in any of these outlets. One of those critical ideas is the principle of peer review.

Peer review is something so fundamental to science that often scientists themselves forget to mention it when explaining what they do to non-scientists, much in the same way a congressman would not think about explaining the fundamentals democracy if someone asked him to explain what he does in his job. This comparison to a politician is apt and I will use it later on, but first let me dispel some misconceptions. The other day I was watching a TV show and as part of the story a "scientist" came and informed the people in a small town that they would have to move because their presence was threatening the local wildlife. As proof of her claim the "scientist" presented her "data" in the form of her doctoral dissertation, which was just a stack of paper in a clear plastic binder. Through out the episode several characters keep referring to the "data" and how it "speaks for itself". Each time someone said something like that my skin began to crawl. Also through out the episode the "scientist" continues to assert that she is right because she has "the data" and she is a "scientist" (it turns out she was wrong, but anyway...). At about that point in the show I about collapsed on the floor in agony.

While scientists do tend to deal with data, the stereotypical scientist that we see in movies or on TV is so far from the truth that for those of us who actually are scientists it can be almost physically painful to watch. One of the things that made the TV show that I mentioned so bad was its complete disregard and ignorance of the concept of peer review, which is something so absolutely fundamental to science that without it science appears no more rational than horoscopes and astrology. To put the concept of peer review in proper perspective I will use an analogy to something we are all familiar with, government.

To put it simply peer review is like holding an election. When a scientist does work they study different phenomena and come up with ideas that explain what they observe. A critical part of the whole process is where the scientists take what they have learned and write it down and present it to others. This point of the process is like a politician campaigning for office, they need the approval of their peers in order to determine the future course of events, or public policy. At this point what the scientist has written is put to intense scrutiny and compared to previous published (peer reviewed) work and the personal experience and expertise of the reviewers. If (IF) what the scientist has written passes peer review then it goes on to be published. In a democracy the only person that gets to hold office is the one that actually gets elected. It is the same with science, the only articles that get publish and are then considered valid science are those that pass peer review. To insist that something is scientifically sound before it passes peer review would be like a politician insisting on moving into his office even before the election.

Once something has passed peer review and gets published, it does not automatically guarantee that it is scientifically sound. We don't declare a politician to be successful before they actually serve their term (despite the opinions of some people). The success of a paper or scientific principle is measured by how influential it is, and whether or not others can duplicate its results. This can be roughly measured by the number of times a paper is cited in other published papers. A large number of citations may indicate a successful paper and good science.

In the case of the TV show I mentioned at the beginning, one reason why its depiction of science was so bad was that the scientist made no mention of her paper being published, peer reviewed and backed up by additional work, all of which are necessary for something to be considered scientifically sound. The character's assertion that she was right because she was a "scientist" is kind of like a politician insisting that they are a democratically elected official because they were appointed by a king. It doesn't make sense and completely undermines the whole concept of democracy, or science as the case may be.

I wish to point out that the principle of peer review is not just a social convention that we use to conduct science. Peer review is useful in preventing bad science, and also in providing a mechanism to construct a useful scientific dialogue, but the reason for it is more fundamental than that. Peer review is an expression of one of the fundamental aspects of the philosophy of science. One of the fundamental driving forces behind science is the concept that everything we observe is independent of the observer. What this means is that if one person observes something then someone else should (or must) be able to observe the same thing. If not then it can not be considered scientific. Another way of putting it is that no one is a privileged observer that has access to knowledge and data that no one else does. This comes from a belief that the universe will always act in a consistent manner and that it does not arbitrarily change the fundamental laws of existence for any one person.

In this sense the principle of peer review is an expression of the fundamental belief that the universe is rational, consistent and does not give special status to any one person. Thus no one can be right because they are a "scientist" (or a preacher, or a teacher, or a politician, or a philosopher etc.), but they are "right" in as much as what they say is in line with reality. Because any one person can be mistaken in what they observe we provide a safe guard in the form of peer review. Thus the principle of peer review is both a statement about a scientists fundamental understanding of how the universe works and a check to make sure that the universe, and also we ourselves are honest and correct in what we have observed.