Thursday, 29 May 2014

Misconceptions About the Universe

Over on youtube, Veritasium has a nice discussion of Misconceptions about the Universe. I  like it, especially as I was the consultant cosmologist :)

A little look down the comments tho, and we see several claims that what Derek says is not correct. Here's a little excerpt
Well, as a cosmologist, I was surprised to read that the Hubble Sphere is an "outdated concept" having seen it used in a professional meeting last week. But let's take a look at the other claims that are made by "fullyawakened" - I must admit they have the lead on me as I am partiallyjetlagged at the moment. As ever, I am going to steal Tamara Davis's standard cosmological picture in a few different sets of coordinates to do this. I've explained these before, but the top one has distance as we know it along the x-axis, and time as we experience it up the y-axis.

"Our observable Universe is getting smaller" is simply wrong. Let's look at the bottom figure, which is in conformal coordinates - basically the expansion has been taken out of the space direction, and the time has been modified so light rays travel at 45 degrees. Objects in the Universe, such as other galaxies, are vertical dotted lines. We are at "now" in the middle. The red lines are our past light cone, and so where the vertical dashed lines cross the red lines, those represent objects that we can see right now.

It doesn't take a lot of thought to see that as "now" moves up the page, the red lines will move up also move upwards, and so the points where they intercept the x-axis will move outwards. As time goes on we will see more and more universe. The cosmic microwave background we see today will be seen to cool and form galaxies into the future, while a more distant cosmic microwave background will take its place. For the eternity of the Universe, the observable Universe will continue to grow, albeit at a slower and slower rate.

As Derek pointed out, there is plenty of stuff outside the Hubble sphere (the yellow shaded area) which we can see; remember, this stuff is moving faster than light with respect to us. And we can see it.

Now, does the Hubble sphere grow or shrink as time goes on. Well, let's look at the top picture, which charts the Hubble sphere (in purple) in physical distance away from us. As you can see, the Hubble sphere is growing and, in our current cosmology, it continues to grow at, again, a slower and slower rate.

Things get more interesting as you look at this in the bottom panel, as if we remove the expansion and look in comoving coordinates, the Hubble sphere starts of growing and then shrinks into the future. But you should understand comoving coordinates before taking too much away from that.

What about the notion that things disappear as they are leaving the observable Universe faster than light? Well, this is not the case, as we've seen that the observable Universe will continue to grow. But things will vanish from sight into the distant future, as per
So, where do they go. I've written about the mapping between experienced time and conformal time in the Future, and one of the nice things about our Universe is that the infinite experienced time becomes a finite conformal time. It means there is one last light cone (the event horizon in the picture). When the dotted line of a galaxy crosses this, this must be the last view that we get of it. But what it means is that in the dim and distant future, the distant galaxies we see will appear to slow down, their rotation will look like it's stopping, exploding stars will take weeks, months, centuries to occur, and as they do, the light from the galaxies will become more and more redshifted, until finally they completely fade from view.

So, fulltawakened, yes, I do do this for a living, and I know what I'm talking about. Ain't the Universe fantastic!

Have cosmologists lost their minds in the multiverse?

Sorry, I've been traveling. I'll be posting soon, but please check out a recent article of mine over at the Conversation called Have Cosmologists Lost their Minds in the Multiverse?


Saturday, 10 May 2014

The outer halo globular cluster system of M31 - I. The final PAndAS catalogue

Back from an exhausting week in Canberra allocating grants for the Australian Research Council. Handing out a few tens of millions of dollars really takes it our of you. The sad part is not the excellent projects we funded, but the many excellent projects we could not as we ran out of cash. Anyway, the meetings are confidential, so I can't discuss them here.

But time to catch up on some of my own science. There is something super exciting coming, so here I'll focus on a merely excellent piece of work by globular cluster finder extraordinaire,  Avon Huxor (excuse his ever so happy face!).

I've written many times about the Pan-Andromeda Archaeological Survey, our superb survey of our nearest neighbour, the Andromeda Galaxy. As well as identifying lots of substructure, the shredded remains of small galaxies, we also see lots of globular clusters, balls of roughly a million stars living together. The Milky Way has about a hundred of these orbiting in the halo. Andromeda has even more.

We've already found a lot of really interesting things about this population of globulars in Andromeda, finding them out to extreme distances, finding them associated with substructure (which means many are immigrants which were born outside) and other excellent things. now it's time to give the data over to the public so they can start to use it. Hence this new paper which present the catalogue of the results.

Some think that science is all about whizz-bang results that "rewrite the textbooks", but science is much more than that. It's all about sharing knowledge, and through sharing it enabling more science to be done. In some areas, typically areas where you can make money (I'm looking at you pharmaceutical companies) data is protected and hidden, but in astronomy there is (generally) a good history of data sharing. We will, in the not too distant future, make our entire PAndAS data-set public. You two can use it if you want to :)

Avon's paper presents the new globular clusters we find. Here's what they look like:
 Cool eh! Importantly, a paper like this tells you a lot more than "these are what we found". It also presents the methods we used to find globular clusters, and how reliable our detections are. This means presenting the mis-detections, the things you initially thought were globulars and then, with more looking, realised they aren't.
And then there's the properties of each cluster, such as its size and brightness, to allow people to look at the properties of the population as a whole.
Then we can compare the properties of the globular around Andromeda to those around the Milky Way
And we find that they are kinda similar, but also not really. Andromeda has more, and they are more widely distributed, and several other things. The Milky Way and Andromeda are very similar, and so why their globular cluster populations differ is a bit of a mystery, is it due to the way they grew, or their current predicament, which is great because it means there's more work to do (keeping food on the table and a speedboat in the harbour [joking about the food!!])! Excellent!

Well done Avon!

The outer halo globular cluster system of M31 - I. The final PAndAS catalogue

We report the discovery of 59 globular clusters (GCs) and two candidate GCs in a search of the halo of M31, primarily via visual inspection of CHFT/MegaCam imagery from the Pan-Andromeda Archaeological Survey (PAndAS). The superior quality of these data also allow us to check the classification of remote objects in the Revised Bologna Catalogue (RBC), plus a subset of GC candidates drawn from SDSS imaging. We identify three additional new GCs from the RBC, and confirm the GC nature of 11 SDSS objects (8 of which appear independently in our remote halo catalogue); the remaining 188 candidates across both lists are either foreground stars or background galaxies. Our new catalogue represents the first uniform census of GCs across the M31 halo - we find clusters to the limit of the PAndAS survey area at projected radii of up to R_proj ~ 150 kpc. Tests using artificial clusters reveal that detection incompleteness cuts in at luminosities below M_V = -6.0; our 50% completeness limit is M_V ~ -4.1. We construct a uniform set of PAndAS photometric measurements for all known GCs outside R_proj = 25$ kpc, and any new GCs within this radius. With these data we update results from Huxor et al. (2011), investigating the luminosity function (LF), colours and effective radii of M31 GCs with a particular focus on the remote halo. We find that the GCLF is clearly bimodal in the outer halo (R_proj > 30 kpc), with the secondary peak at M_V ~ -5.5. We argue that the GCs in this peak have most likely been accreted along with their host dwarf galaxies. Notwithstanding, we also find, as in previous surveys, a substantial number of GCs with above-average luminosity in the outer M31 halo - a population with no clear counterpart in the Milky Way.

Saturday, 3 May 2014

How elusive is dark matter?

Your mind can wander into some strange areas when lying in bed in the early hours of the morning, in the quiet before the kookaburras start laughing their heads off.

When we read about dark matter, it often easy to think that it's out there, far away from Earth, like black holes, neutron stars, and little green men. But it's important to remember that the orbit of the Sun around the Galaxy depends upon not only the stars we see, but the dark matter we don't.

In fact, we have to remember that dark matter surrounds and threads our Solar System. But just how much?

Anyone who has done any university physics knows that within the Solar System we can happily use Newton's laws of gravitation and laws of motion to calculate the paths of the planets, with the Sun as the massively dominant source of gravitational pull, with the planets just providing additional little tugs. If there is dark matter in the Solar System, it seems to be irrelevant.
Just how much dark matter is there then?

By looking at the motions of stars, we can estimate how much dark matter there is. The density reported by this study is that within the Solar System is
This is in units of Solar masses (which are big) and parsecs (which are also big). What's this in "normal" units (and for our american cousins, I mean metric).

The wonderful Wolfram Alpha tells us that 1 Solar Mass, spread over a cubic parsec is
and the local dark matter density is

Ouch! This is small. No wonder we don't have to worry about the dark matter in the Solar System.

So, if dark matter is made of particles, how massive would they be? Well, there's a lot of speculation on this, but let's take a look at some recent suggestions that it's about 30 billion electron volts. Huh, you say, how is that a mass? In science, you choose units that are useful to you. In astronomy, we use Solar masses, as that makes it easier to talk about stars (it's a pain in kgs!), and in particle physics, they use electron volts.
 How big is an electron volt? Well, wikipedia tells us that it's
which is pretty small, and so 30 billion of these are
Ooooh - this is getting interesting. This means that there are roughly 10,000 dark matter particles per cubic metre in the Solar System. And don't forget that the room in which you sit, or field in which you are lying down in, are in the Solar System. And in every cubic metre about you there are roughly 10,000 dark matter particles, about 10 per litre.

When you reach this point, cup your hands together. In there there is probably a dark matter particle or two. Dark matter doesn't seem remotely as elusive now, does it.