Sunday, 13 April 2014

Gravitational lensing in WDM cosmologies: The cross section for giant arcs

We've had a pretty cool paper accepted for publication in the Monthly Notices of the Royal Astronomical Society  which tackles a big question in astronomy, namely what is the temperature of dark matter. Huh, you might say "temperature", what do you mean by "temperature"? I will explain.

The paper is by Hareth Mahdi, a PhD student at the Sydney Institute for Astronomy. Hareth's expertise is in gravitational lensing, using the huge amounts of mass in galaxy clusters to magnify the view of the distant Universe. Gravitational lenses are amongst the most beautiful things in all of astronomy. For example:
Working out how strong the lensing effect is reveals the amount of mass in the cluster, showing that there is a lot of dark matter present.

Hareth's focus is not "real" clusters, but clusters in "synthetic" universes, universes we generate inside supercomputers. The synthetic universes look as nice as the real ones; here's one someone made earlier (than you Blue Peter).

 Of course, in a synthetic universe, we control everything, such as the laws of physics and the nature of dark matter.

Dark matter is typically treated as being cold, meaning that the particles that make up dark matter move at speeds much lower than the speed to light. But we can also consider hot dark matter, which travels at speeds close to the speed of light, or warm dark matter, which moves at speeds somewhere in between.

What's the effect of changing the temperature of dark matter? Here's an illustration
With cold at the top, warmer in the middle, and hottest at the bottom. And what you can see is that as we wind up the temperature, the small scale structure in the cluster gets washed out. Some think that warm dark matter might be the solution to missing satellite problem.

Hareth's had two samples of clusters, some from cold dark matter universes and some from warm, and he calculated the strength of gravitational lensing in both. The goal is to see if changing to warm dark matter can help fix another problem in astronomy, namely that the clusters we observe seem to be more efficient at producing lensed images than the ones we have in our simulated universes.

We can get some pictures of the lensing strengths of these clusters, which looks like this
This shows the mass distributions in cold dark matter universes, with a corresponding cluster in the warm dark matter universe. Because the simulations were set up with similar initial conditions, these are the same clusters seen in the two universe.

You can already see that there are some differences, but what about lensing efficiency? There are a few ways to characterise this, but one way is the cross-section to lensing. When we compare the two cosmologies, we get the following:

There is a rough one-to-one relationship, but notice that the warm dark matter clusters sit mainly above the black line. This means that the warm dark matter clusters are more efficient at lensing than their cold dark matter colleagues.

This is actually an unexpected result. Naively, we would expect warm dark matter to remove structure and make clusters puffy, and hence less efficient at lensing. So what is happening?

It took a bit of detective work, but we tracked it down. Yes, in warm dark matter clusters, the small scale structure is wiped out, but where does the mass go? It actually goes in to the larger mass halo, making them more efficient at lensing. Slightly bizarre, but it does mean that we have a way, if we can measure enough real clusters, it could give us a test of the temperature of dark matter!

But alas, even though the efficiency is stronger with warm dark matter, it is not strong enough to fix the lensing efficiency problem. As ever, there is more work to do, and I'll report it here.

Until then, well done Hareth!

Gravitational lensing in WDM cosmologies: The cross section for giant arcs

The nature of the dark sector of the Universe remains one of the outstanding problems in modern cosmology, with the search for new observational probes guiding the development of the next generation of observational facilities. Clues come from tension between the predictions from {\Lambda}CDM and observations of gravitationally lensed galaxies. Previous studies showed that galaxy clusters in the {\Lambda}CDM are not strong enough to reproduce the observed number of lensed arcs. This work aims to constrain the warm dark matter cosmologies by means of the lensing efficiency of galaxy clusters drawn from these alternative models. The lensing characteristics of two samples of simulated clusters in the warm dark matter ({\Lambda}WDM) and cold dark matter ({\Lambda}CDM) cosmologies have been studied. The results show that even though the CDM clusters are more centrally concentrated and contain more substructures, the WDM clusters have slightly higher lensing efficiency than their CDM counterparts. The key difference is that WDM clusters have more extended and more massive subhaloes than CDM analogues. These massive substructures significantly stretch the critical lines and caustics and hence they boost the lensing efficiency of the host halo. Despite the increase in the lensing efficiency due to the contribution of massive substructures in the WDM clusters, this is not enough to resolve the arc statistics problem.

Sunday, 6 April 2014

The Multiverse is not not the answer!

With the recent BICEP2 results, the question of the Multiverse has raised it's less than pretty head. The Multiverse is a topic which polarizes people, some who embrace the idea that our Universe is just one of countless trillions and trillions and trillions of other universes out there. Others get a little cross with the popularity of the topic. And then there are those (looking at you slashdot commenters) who generally talk nonsense.
Over at Starts with a Bang Ethan Siegal tells us that The Multiverse is not the answer. The essential crux of Ethan's argument is that, in calling on the Multiverse, cosmologists are giving up on science. What does he mean?

The Multiverse is not a single concept, there is more than one kind, but the basic idea is that, as well as our Universe, which we find quite cosy for life, there are a myriad of other universes. These universes are not identical to our own, but possess different physics, different masses of the fundamental particles, different strengths of the fundamental forces, different forces even. 

I won't go into detail here (as I am going to go into detail elsewhere - watch this space!) but most of these universes would be sterile, completely devoid of life. But aren't we lucky to find ourselves in a universe in which we can live? Of course, we have to find ourselves in such a universe, otherwise we would not be here to ask the question. This is the Anthropic Principle (a concept more wildly and crazily discussed than the Multiverse).
So, what's Ethan's problem? Well, if are going to think that we are just one of this myriad of universes, then there is no point asking any questions about why the electron has the mass it does, or why gravity is weaker than the weak force. It's just the roll of the cosmological dice that gave these values, and these values are just right for us to live comfortably. Calling on the Multiverse is akin to saying "God did it" and so it not science. We can pack-up our cosmological bag, and go home.

But I don't think Ethan is correct. As he points out in his article, we still have mysteries to solve. One of the most pressing is to get gravity and the other forces to play nicely together in extreme conditions of the early Universe. This could be the long sought after theory of quantum gravity which will allow us to see back into the creation of the Universe. Some think that the constants of nature will fall out of this ultimate theory, as we will know why an electron has the mass it does.

But there are other things we need to understand, such as the details of inflation, minor things like how it started, how it stopped, and, well, most other things about it. We could end up with several competing theories that describe our inflating universe, and producing multiple universes as it goes. The differing models may imprint themselves on our observations, and we can tell them apart, but we may never see the other universes out there as they are beyond our horizon.
Your multiverse generating model does, however, have a big hurdle to overcome, and that is that it must produce at least one universe, one out of the potential bazzion universes it can produce, that is the Universe, the one we find ourselves living in. If your model cannot account for our cosmic home, then off to the rather large and overflowing dustbin of science. 

So, I don't think anyone is giving up. I don't think cosmology is moving from science into pseudo or non-science. And while we may never be able to see the sibling universes all around us, our mathematical picture of the evolution of the multiverse is still testable, and is still science. Freaky science, but science all the same. 

There's a lot more to come on this topic, so watch this space.

Monday, 24 March 2014

The PAndAS Field of Streams: stellar structures in the Milky Way halo toward Andromeda and Triangulum

A cool paper this week, written by long-time collaborator, Nicolas F. Martin. Again, it's a PAndAS paper but the focus is not Andromeda but our very own Milky Way. How, I hear you say.

The starting point is the Field of Streams, a truly wonderful image of a massive area of sky taken as part of the Sloan Digital Sky Survey. Here it is
The Sloan Survey had many goals, but one of those was to look at the stars in our own Galaxy, especially in the Galactic Halo. If our picture of how galaxies form and grow are correct, basically by eating smaller systems, we should expect the halo to be full of the debris from things being consumed at the present time. And this is what we see in the Field of Streams.

The big swoop across the image is the tidal debris from the Sagittarius Dwarf Galaxy, which, even though it was only discovered in 1994, completely wraps the Galaxy. There's lots of other debris in there too, including the Monoceros Ring which wraps the outer parts of the disk we inhabit.

So, looking towards Andromeda, we also see all of the stars within our own Galaxy halo. What do we see? Well, it's easy to see in what Nicolas refers to as the Money plot (I'm not sure if he knew what the origin of this phrase actually is). Anyway, here's our new PAndAS Field of Streams.
The big black ellipse is where Andromeda is, and the smaller one is M33, the Triangulum Galaxy. The outer white line is the "footprint" of the PAndAS survey. To give you an idea of how big the survey is, the distance between Andromeda and Triangulum is 15 degrees, which is 30 times the size of the full Moon.

What we've for here are stars that we've selected as being in the halo of the Milky Way, colour-coded by distance. And what we see is not a boring smooth distribution, but streams! Yay! So towards Andromeda is our own Field of Streams! In fact, it's a bit of a mess. Here's what we've got.

Lots and lots of stuff. The green hashed areas are big blobs of stars which are streamy, and the blue hashed area is another chunk of stars. The thin streak is a globular cluster stream, tidal debris from an individual cluster being destroyed; we're starting to see a growing number of these.

So, what have we learnt? Well, PAndAS is much smaller than the Sloan Digital Sky Survey, covering only ~1% of the sky, but we go significantly deeper. And we see lots and lots of streams, which suggests that if Sloan went as deep across the huge amount of sky that it can see, the field of streams would be absolutely amazing!

And the good news is that the next generation of telescope is going to do this. One of the best is the Large Synoptic Survey Telescope which is going to look at the sky over and over again, observing deeper and deeper. It will be on sky around 2021, and then will observe for about 10 years. I can't wait to see what the sky is going to look like, but I am sure it will be lovely.

But for now, well done Nicolas!

The PAndAS Field of Streams: stellar structures in the Milky Way halo toward Andromeda and Triangulum

We reveal the highly structured nature of the Milky Way stellar halo within the footprint of the PAndAS photometric survey from blue main sequence and main sequence turn-off stars. We map no fewer than five stellar structures within a heliocentric range of ~5 to 30 kpc. Some of these are known (the Monoceros Ring, the Pisces/Triangulum globular cluster stream), but we also uncover three well-defined stellar structures that could be, at least partly, responsible for the so-called Triangulum/Andromeda and Triangulum/Andromeda 2 features. In particular, we trace a new faint stellar stream located at a heliocentric distance of ~17 kpc. With a surface brightness of \Sigma_V ~ 32-32.5 mag/arcsec^2, it follows an orbit that is almost parallel to the Galactic plane north of M31 and has so far eluded surveys of the Milky Way halo as these tend to steer away from regions dominated by the Galactic disk. Investigating our follow-up spectroscopic observations of PAndAS, we serendipitously uncover a radial velocity signature from stars that have colors and magnitudes compatible with the stream. From the velocity of eight likely member stars, we show that this stellar structure is dynamically cold, with an unresolved velocity dispersion that is lower than 7.1 km/s at the 90-percent confidence level. Along with the width of the stream (300-650 pc), its dynamics points to a dwarf-galaxy-accretion origin. The numerous stellar structures we can map in the Milky Way stellar halo between 5 and 30 kpc and their varying morphology is a testament to the complex nature of the stellar halo at these intermediate distances.

Tuesday, 18 March 2014

Bad Astronomy: The Expanding Universe

I'm not going to join the chatter on the BICEP2 result as there's already a lot of commentary out there (although ABC News 24 seem to have missed it, so no chance to mess it up :)

So, a tiny post today. Here's a picture from the BBC story.

This is supposed to be the Big Bang, with galaxies rushing from a  central explosion.

I've written about this before, but this is not what the Big Bang was. It was not an explosion in preexisting space.

If we take the cosmological parameters as we currently know then (and assume a simple topology) then the Universe is now infinite in extent. And it was a billion years ago, and it was 13.5 billion years ago, and it was throughout the history of the Universe.

When the Universe was a 100 million billion degrees celsius, it was this temperature everywhere throughout the infinite Universe. As it cooled down, it cooled down everywhere in the infinite Universe. When it got to a few hundred thousand years after the Big Bang and the plasma neutralized, and the radiation which became the CMB could flow freely through the Universe, this happened everywhere in this infinite Universe.

It was not an explosion.

I know I bang on about this, but hearing the media stumbling over this, unable to understand the meaning of things like BICEP2 because they don't get the basics of the Big Bang, gets a little depressing.

Monday, 17 March 2014

Stupid Americans: 1 in 4 think the Sun goes around the Earth

I am going to avoid the breathless headlines doing the rounds about an upcoming press release about the detection of cosmological gravitational waves. Firstly, a press release about an upcoming press release is a seriously poor way to do science. And I am already groaning internally about how poorly any result will be presented in the media (looking at you ABC News 24, especially Michael Rowland, who still calls the Higgs boson the Higgs bosun).

So, back to something we all enjoy, America bashing (every year I eagerly await the letters in the newspapers complaining about halloween in Australia, telling us that we don't want this American rubbish here while chewing down on a McDonalds, sipping coke and driving a ford, stopping at the American stop sign before rushing home to watch the latest American cop show on TV). And before any American readers get riled up, the preceding text is an example of sarcasm.

Apparently, 1 in 4 Americans think that the Sun goes around the Earth. How can they be so stupid? Ha ha ha... Sorry, what was that? In Europe it's 1 in 3... Oh... Well, here in Australia they are bound to be smarter? ... Surely... apparently not.

There has been a little bit of wailing and gnashing of teeth over this. What's going on with the world, how did people miss the Copernican revolution? Is science in trouble?

Hang on.. Let's remember that the Copernican revolution was only a few hundred years ago and for the majority of human history people, lots and lots of people, from the not so bright to the smartest of the smart thoughts that the Sun went around the Earth.

Why? Because sitting here on Earth, that is what it appears to do. It comes up at one horizon, trundles across the sky, and disappears over the opposite horizon. And the next day, it does it all over again. The Egyptians saw this, as did the Greek, Romans etc. But, you say, they were ignorant of the truth!

Well, I think all this angst is misplaced. I think it's perfectly fine to think that the Sun goes around the Earth. You may think that I've gone crazy, but let me explain.

What do we mean when we say that one thing goes around another? Well, I'm sure there is more than one way to define it, but I'm going to say object A goes around object B if it changes changes its angle as viewed from B, tracing out 360 degrees.

As seen from the Sun, this is what the Earth does, tracing out a complete path in 365 days. The Earth goes around the Sun.

But what if I centre my coordinate system on me, and then trace out the path of the Sun in my coordinates, then the Sun is above my head at noon, below my feet twelve hours later, and then is back above my head after another 12 hours. In my coordinate system, the Sun goes around the Earth.

I know that you are stomping your feet and saying "but your coordinate system is rotating because it's pinned to the surface of the rotating Earth. It only appears that the Sun goes around the Earth".

Of course, I know this. But the question people were asked was

Not does "Does the Earth orbit the Sun, or does the Sun orbit the Earth?", but "go around" and I think it is completely legitimate to say that either is true. As viewed from the Earth, the Sun goes around the Earth. Viewed from the Sun, the Earth goes around it. 

People are not as "stupid" as people think. And the people who did the survey should have thought a little harder about the question.

Oh, and the answer to  "Does the Earth orbit the Sun, or does the Sun orbit the Earth?" is neither, both orbit the common centre of mass. How many of you would have gotten that correct?

Wednesday, 12 March 2014

A thousand shadows of Andromeda: rotating planes of satellites in the Millennium-II cosmological simulation

Before we start, well done to the Running Astronomer on her most recent race.

Papers are like buses. Well, their not really, but hot on the heels of yesterdays cosmology paper, another paper on the arxiv, this time with the friendly title of A thousand shadows of Andromeda: rotating planes of satellites in the Millennium-II cosmological simulation. This is a cool paper and harks back to our discovery last year of the discovery of a strange corotating plane of satellites in the Andromeda Galaxy.

Well, here we are more than a year later, and there is still no satisfactory answer to what it is doing there (or the equally strange, and badly named, Vast Polar Structure of the Milky Way).

Over the last few decades, we've built up a complex model for the formation and evolution of structure in the Universe, and the favourite picture, know as ΛCDM, works extremely well on large scales.
The Λ part refers to the influence of dark energy, where as the CDM = Cold Dark Matter is the influence of dark matter.

It is quite amazing how well this over all picture works, but we know where it falls over. It falls over on the small scale, on the scale of individual galaxy halos.

I've written about the missing satellite problem, the fact that our models of galaxy formation predict that a galaxy like our own Milky Way should be accompanied by hundreds and thousands of smaller galaxies, while when we look on the sky we see a few dozen.

But there is a slippery aspect to studying galaxy formation, and that is the complexity of gas physics. Good old dark matter is nice, simple and well behaved. Gas, however, can cool, collapse, form stars, go supernova, and, well, just be bloody messy. And such messiness is embodied in complicated mathematics (some of which are approximations as you are trying to study the motions, temperatures and densities of gas on scales much smaller than your simulation can resolve).

With this complexity, you can invoke gas physics in the early universe to blow away gas, basically killing the star formation in many dwarf galaxies dead. So, with this, we are surrounded by thousands of stillborn dwarfs. We just can't see them.

Not everyone is satisfied with this answer and wonder if there is something more fundamentally wrong with physics that needs to be fixed up.

The plane of satellite galaxies is another complexity that our cosmological model has to explain.

When I give talks on this, I get a few hand-wavy dismissals that the complexity of gas physics or accretion could explain this weird plane, and there is nothing to worry about, and everything is lovely with the world.

This is where this new paper comes in. Written by long time collaborator, Rodrigo Ibata, we take a look at one of our best computer models for galaxies like our own Milky Way. This is the Millennium Simulation of structure formation, which is a massive computer simulation which is open to all to play with; if you find yourself with the lose end, I recommend that you do, and you too may find something cool about the Universe.

So, what do we do? Well, we look at the satellites in the galaxies, and ask "What are their properties?", and importantly, "Do they look anything like the observed plane of satellites?". Essentially, we want to see if we can find a significant planes as thin as the one we see in Andromeda *and* has the correction rotational signature.

So, we spend a lot of time selecting out planes of satellites, and there's lots and lots of them. Chuck down enough (almost) random points, and you will find nice planes.

But the big question, the important question, is how many of the planes are moving together. A little statistical machinations later, what do we find. We find....
after slicing to extract the rotating planes, we find... we find... drum roll....

Planes like we the one we observe are extremely rare (pretty close to impossible) if we want both the positions and velocities to match.

Of course, this is assuming that there the entire ΛCDM idea is correct, and we take it at face value. Maybe somewhere we are missing the additional complexity through the physics of gas and messy accretion, and maybe everything as wonderful as such planes are everywhere. But sometimes brushing problems under the carpet is a little like griping onto a security blanket. And if you are planning to give me that hand wave I get in talks, the one that suggests the world is wonderful and that ΛCDM explains all, remember I will be thinking that it tells me more about you than what I am talking about.

But other than that, well done Rod!

A thousand shadows of Andromeda: rotating planes of satellites in the Millennium-II cosmological simulation

In a recent contribution, Bahl \& Baumgardt investigated the incidence of planar alignments of satellite galaxies in the Millennium-II simulation, and concluded that vast thin planes of dwarf galaxies, similar to that observed in the Andromeda galaxy (M31), occur frequently by chance in Λ-Cold Dark Matter cosmology. However, their analysis did not capture the essential fact that the observed alignment is simultaneously radially extended, yet thin, and kinematically unusual. With the caveat that the Millennium-II simulation may not have sufficient mass resolution to identify confidently simulacra of low-luminosity dwarf galaxies, we re-examine that simulation for planar structures, using the same method as employed by Ibata et al. (2013) on the real M31 satellites. We find that 0.04\% of host galaxies display satellite alignments that are at least as extreme as the observations, when we consider their extent, thickness and number of members rotating in the same sense. We further investigate the angular momentum properties of the co-planar satellites, and find that the median of the specific angular momentum derived from the line of sight velocities in the real M31 structure (1.3×104 km/s kpc) is very high compared to systems drawn from the simulations. This analysis confirms that it is highly unlikely that the observed structure around the Andromeda galaxy is due to a chance occurrence. Interestingly, the few extreme systems that are similar to M31 arise from the accretion of a massive sub-halo with its own spatially-concentrated entourage of orphan satellites.

Tuesday, 11 March 2014

Non-linear Chaplygin Gas Cosmologies

Ultra-quick post today, but a new paper on The Arxiv. The title is "Non-linear Chaplygin Gas Cosmologies", catchy eh! But what does it mean?

Essentially, we have two big dark mysteries in the universe, dark matter and dark energy, and what we want to do is try and reduce the number of mysteries by a factor of two. We can do this with a Chaplygin gas, modifying its usual properties so that on small scales it looks like dark matter, while on large scales it acts as dark energy.

In this first paper, we basically lay out our new model, led by Pedro Avelino who was visiting the Sydney Institute for Astronomy from Portugal. It's a bit mathematical, but in coming papers we will detail a number of observational tests we compare the model  to.

That is yet to come, and I will write a lot more detail then, but I have to run. Before I go, I'll note that this is my first paper with Krzysztof Bolejko, and so my Erdos Number has significantly shrunk. More on that too!

But for now, well done Pedro!

Non-linear Chaplygin Gas Cosmologies

We study the non-linear regime of Unified Dark Energy models, using Generalized Chaplygin Gas cosmologies as a representative example, and introduce a new parameter characterizing the level of small scale clustering in these scenarios. We show that viable Generalized Chaplygin Gas cosmologies, consistent with the most recent observational constraints, may be constructed for any value of the Generalized Chaplygin Gas parameter by considering models with a sufficiently high level of non-linear clustering.