Category Archives: Astronomy

Solid smoke and stardust

Recently I visited the Canberra Deep Space Communication Complex at Tidbinbilla in the Australian Capital Territory, where they have a sample of aerogel, aka “solid smoke”.

The author in front of the Big Dish at the Canberra Deep Space Communication Complex
Here’s me posing in front of the Big Dish at Tidbinbilla. Disappointingly, their cafe doesn’t have a meal called a ‘Big Dish’. Get onto that, CDSCC!

This ethereal substance was used on NASA’s Stardust probe to capture dust from comet Wild 2 and return it to Earth—along with a handful of precious samples of interstellar dust, giving a rare glimpse of material from outside our Solar System.

A 2 gram piece of aerogel holding up a 2.5 kg brick (image from NASA/JPL-Caltech)
A 2 gram piece of aerogel holding up a 2.5 kg brick (image from NASA/JPL-Caltech)

Listen to our show from 27 August 2015 to hear all about it.

You can also hear our interview with British scientist Maggie Aderin-Pocock, host of the long-running astronomy TV show The Sky at Night, and who’s also known for answering important questions like, do we really need the Moon?

Yes. Yes we do.

Doppler affects you and me, quite frequently

It’s making the news in oceans both Indian and Saturnian, tracking the movements of space probes and missing Malaysian airliners. And yet you encounter it every day, when you hear a car passing you on the road change from high to low pitch. So what exactly is the Doppler effect, and how does it work?

(Q: What sound does a cat make when it goes past at high speed? A: Meeeeeeeeeeeeeeeowwwww.)

As you might expect, the Doppler effect was named after the Christian Doppler, an Austrian physicist—although he only became a physicist because he was too frail to enter his father’s stonemason business—who proposed it in Prague in 1842.

It happens whenever there is movement relative to a source that’s emitting waves, whether they’re light, sound, water or something else. In the case of the moving car, think of its soundwaves as a series of peaks and troughs. The car emits one wave, i.e. one peak, and then another about 1 millisecond later.

But in that millisecond the car has moved closer to you, so the second peak has less distance to travel. It therefore reaches you less than 1 millisecond after the first peak does. This means that for you each peak is separated by less than a millisecond, so you hear the sound at a higher frequency.

OK, that maybe a little hard to picture, so try it visually instead. Imagine the waves as concentric rings being emitted by the source, they bunch up at the front and stretch out behind it. Or don’t imagine it: look at the picture below.

Doppler effect showing circular wave fronts emitted from a source moving to the right
Doppler effect from a source moving at 0.7 the speed of wave propagation (Image by Lookang with many thanks to Fu-Kwun Hwang and author of Easy Java Simulation Francisco Esquembre, via Wikimedia Commons)

However you imagine it, the frequency change due to the Doppler effect makes a very convenient way to measure velocity, so it has many applications. Talking about moving cars, well it’s the Doppler effect that the police radar uses to tell whether you’re speeding (see the NSW Police Radar Manual [PDF 4.3 MB]).

It’s also famously what we use to measure the expansion of the universe. When a light source like a star or a galaxy is moving away from us, the electromagnetic waves it emits go to the low frequency or red end of the spectrum, so we say it’s red-shifted. If it’s coming towards us, it’s blue-shifted. By measuring the redshift of galaxies depending on how far away they are from us, we can calculate how fast the universe is expanding (due to the expansion of the universe, the further something is, the faster it is moving away).

But if understanding the history of the universe isn’t enough, the Doppler effect still makes the news; specifically, in the hunt for missing Malaysian Airlines flight MH370.

Using what the BBC called “cutting-edge methods”, the British satellite firm Inmarsat received radio pings from the missing plane, and by comparing how the frequency of the signal differed from what it’s supposed to be when it’s transmitted, they could work out how the plane was moving. That’s how they determined it flew to  the Southern Indian Ocean, where the search is currently focussed.

Diagram showing how by triangulating the pings from the MH 370 with a calculation of its speed as determined by the Doppler effect, it was possible to calculate the aircraft's path

The other bit of recent Doppler effect news was the discovery of an ocean under the icy surface of Enceladus, a moon of Saturn. Again, the scientists used changes in the frequency of radio signals, this time from the spacecraft Cassini, which was flying past it (Iess L, Stevenson DJ, Parisi M, Hemingway D, Jacobson RA, Lunine JI, Nimmo F, Armstrong JW, Asmar SW, Ducci M & Tortora P 2014, “The gravity field and interior structure of Enceladus”, Science, vol. 344, no. 6179, pp. 78–80, DOI: 10.1126/science.1250551).

By looking at how Cassini’s speed changed as it flew past Enceladus, they could determine the forces of gravity acting on it, which in turn allowed them to calculate the distribution of mass inside the moon. These were changes in speed of mere millimetres per second, but allowed them to figure out there was liquid water—which is denser than ice—and a relatively light rocky core.

Cross-section image of Saturn's moon Enceladus, showing its rocky core and liquid ocean at the southern pole, emitting geysers through the icy crust
Diagram of the theorised interior of Saturn’s moon Enceladus, based on measurements by NASA’s Cassini spacecraft and NASA’s Deep Space Network. The gravity measurements suggest an ice outer shell and a low density, rocky core with a liquid water ocean sandwiched in between. This is also responsible for the plumes of water vapour shown at the moon’s South Pole (Image by NASA/JPL-Caltech)

So it may be commonplace, everyday science, but it’s good to see the Doppler effect is still making waves after all these years.

Massive, supermassive and superdupermassive black holes

There’s a monster lurking in the middle of our galaxy. You might not be able to see it, but we know it’s there. Its diameter is 10 times that of the Sun, but its mass is 4 million times. It’s what we call a supermassive black hole.

OK, it’s 27,000 light years away, so it’s probably not going to get you, but still: a supermassive black hole. Let that sink in, so to speak.

‘Normal’ black holes sound pretty massive themselves. If a star is bigger than about 3 times the mass of the Sun, then eventually it reaches a point where it can no longer hold up under its own weight, and it collapses into an object with gravity so strong that even light cannot escape. These are called stellar black holes.

The biggest stellar black hole so far confirmed is about 16 solar masses, but there are indications they can get up to around 33 solar masses.

However, the black holes believed to be at the centres of most galaxies are much, much bigger: more than 100,000 times the mass of the Sun. Hence the label supermassive black holes.

The location of our galaxy's supermassive black hole, hidden behind opaque dusk in the Milky Way, in the constellation Sagittarius. An inset shows a photo of it taken in the X-ray spectrum (click to embiggen)
Our galaxy’s supermassive black hole is hidden behind opaque dusk in the Milky Way, in the constellation Sagittarius. It can’t be seen in visible light, but it can be seen in the radio or X-ray spectrum, as seen here in the inset photo taken by NASA’s Chandra X-ray Observatory. Click to see a bigger image (Photo sources Moondigger and NASA/CXC/MIT/F. Baganoff, R. Shcherbakov et al., via Wikimedia Commons)

So if there’s something that big in our galaxy, then why can’t we see it? Well, between it and us there’s an awful lot of stuff.

You’ve probably seen the Milky Way in the sky, a cloudy band visible at night when you’re well away from the city. That’s the main plane of our galaxy. If you could stand outside and away from it, you’d see that it’s a spiral galaxy, i.e. a sort of disc shape made of four swirling arms, with a pronounced bulge in the centre.

From the inside, you just see a cloudy band stretching across the sky, with a lot of opaque dust and gas blocking out the good bits like the dense middle. But it’s there alright, in or near the constellation Sagittarius (see the picture above).

Even though we can’t see it directly – at least not with visible light – we can detect it with radio waves. And in the radio spectrum we see a very, very powerful radio source called Sagittarius A*. The radio waves are believed to be electromagnetic radiation given off from the accretion disk of the black hole: that’s where things spin around it really, really fast before they fall in. And when charged particles spin around fast like that they give off electromagnetic radiation (which actually means they lose energy and so fall in even faster. Not a good idea perhaps, but you can’t fight physics).

So we can see the radio waves, but how do we know Sagittarius A* is a black hole? Well, we can also detect 28 other stars orbiting it. One of them, called simply S2, orbits every 15.2 years and gets as close as 122 times the distance from the Earth to the Sun.

From the speed and distance of S2, we can calculate that the object in question has a mass of about 4.1 million times the mass of the Sun. That much mass in that small a volume has to be a black hole.

Its dimensions are given by something called the Schwarzschild Radius, which tells us that the black hole’s event horizon – the point at which light is no longer able to escape – is at about 13.3 million kilometres. That’s only about 10 times the diameter of the Sun, or 9% of the distance from the Sun to the Earth.

And yet it has a mass 4 million times that of the Sun. For comparison, the Sun is 333,000 times the mass of the Earth. The difference between the black hole and the Earth is the same as that between you and a grain of pollen.

Even so, there are bigger black holes out there. Much, much bigger (you can see where this is going).

Recently, one with a mass of 17 billion suns was discovered in a galaxy only 250 million light years away (van den Bosch RCE, Gebhardt K, Gültekin K, van de Ven G, van der Wel A, Walsh JL 2012, “An over-massive black hole in the compact lenticular galaxy NGC1277”, Nature, vol. 491, no. 7426, pp. 729-731, doi:10.1038/nature11592, arXiv:1211.6429v1 [astro-ph.CO]).

I call it a superdupermassive black hole, although the authors called it ‘over-massive’.

This term is actually appropriate, because it’s much larger compared to its host galaxy than previously discovered black holes. Although small in comparison, our galaxy is in more typical proportion, with the central black hole being 0.1% the mass of all other stars. But the black hole in NGC1277 is 14% of its galaxy’s stellar mass.

The animation embedded below shows how the black hole was identified, using measurements of stars in the galaxy to calculate their orbits and hence the mass at their centre. The photo in the background was taken by the Hubble Space Telescope (NASA/ESA/Fabian/Remco C. E. van den Bosch MPIA).

But even though it’s so big, this superdupermassive black hole isn’t a unique freak. The researchers have also found five other galaxies with similar extreme proportions. Instead, it suggests we may need to rethink our theories of how galaxies form. After all, we’ve been using our own galaxy as a typical example, but there seems to be a much bigger and more complex variety.

What we can say for certain is that it shows what huge objects are out there in the universe. Much too huge for our puny human adjectives.

I spoke to Professor Rachel Webster from the University of Melbourne about this discovery, on our show that aired on 13 December 2013. You can listen to the podcast.

A transcript follows after the break…

Continue reading Massive, supermassive and superdupermassive black holes

Rocks from the sky

Why do people talk about a ‘meteoric rise’, when what meteors do is fall? Very odd.

Recently I had the great pleasure of visiting Meteor Crater, Arizona, an enormous hole in the ground that’s not far from another famous hole in the ground, the Grand Canyon. If you ever visit the latter, I highly recommend taking a slight detour to see the former.

Meteor Crater – a very matter-of-fact name – was formed about 50,000 years ago by a meteorite 50 metres across and weighing around 300,000 tons, releasing 10-20 megatons of TNT. That’s about the same force as a hydrogen bomb.

Yours truly standing next to Meteor Crater, Arizona, a hole in the ground 1.2 km across and 170 m deep (click to embiggen)
Yours truly at Meteor Crater, Arizona. This hole in the ground is 1.2 km across, 170 m deep and was formed by an iron-nickel meteorite weighing around 300,000 tonnes that hit the ground 50,000 years ago, with an explosion equivalent to releasing 10-20 megatons of TNT, or one (1) hydrogen bomb.

The crater was known to Native Americans, but the earliest historical record of it is from 1871. Early studies concluded that it was a volcano, but in 1902, meteorite-proponent Daniel Moreau Barringer acquired it, with an intent to mine it for iron.

The idea that rocks or anything else could fall from the sky was initially very controversial. People laughed at German physicist Ernst Florens Chladni, who in 1794 was the first person to suggest the idea. But he was vindicated in 1795, when a 25 kilogram stone fell in broad daylight in Wold Cottage England. Some were still sceptical though, notably the French Academy of Sciences; until 1803, when about 3,000 meteorites fell on France. That pretty much shut them up.

Now it’s important to get some terminology right here: a meteorite is the rock or mineral that actually hits the ground. Meteor is what it’s called when it’s flying through the sky, aka a shooting star. And when it’s merely a rock floating in space, it’s a meteoroid, unless it’s really big, in which case it’s an asteroid.

Barringer was far from the first to try and exploit meteorites for their iron. Before people learned how to smelter ores they were the main source of iron for tools and the like.

Overall though, fewer than 10% of meteorites are iron-nickel like the one that hit Arizona, or that formed the Wolfe Creek Crater in Western Australia (0.87 kilometres across and up to 300,000 years old, one of 27 Australian meteorite craters listed in the Earth Impact Database, mostly in Western Australia and the Northern Territory). They’re believed to come from cores of asteroids that have broken up, which explains their rarity.

Most other meteorites are a stony material called chondrite, made of small round particles called chondrules. These are thought to be rock formed at the birth of the Solar System.

Some (about 4.6%) contain carbon and are known as carbonaceous chondrites. These sometimes include organic compound and are eagerly sought, like the one that hit California on 22 April 2012.

With so many rocks bombarding the planet, and giant impact craters like that in Arizona, they’ve got to be pretty dangerous, right?

Continue reading Rocks from the sky

WIMPs may occupy the universe

As I type this post, the physics community is holding its collective breath waiting for news from the Large Hadron Collider, in the hope that it may give us a clue for the next step in understanding how the universe works. In the meantime, any unusual result like possible faster than light neutrinos is seized in the hope of finding something new.

Despite this, I can’t help wondering if we’re missing an obvious piece of the puzzle: dark matter and dark energy together account for about 96% of the total mass of the universe.

So if everything we can see and experiment on is a mere 4% of all that there is, is there any wonder it doesn’t make sense?

Okay then, do we know anything at all about what dark matter may be? Well, for a long time the leading candidates have been either MACHOs or WIMPs. Yes, those are the actual names.

MACHO stands for Massive Astrophysical Compact Halo Object, and it basically refers to clumps of normal matter that sit in the “haloes” around galaxies, but for some reason can’t be seen. They could be objects like planets, brown dwarfs (unignited stars) or even black holes.

Although MACHOs don’t emit enough light for us to see, they can be detected by gravitational microlensing, which is when their gravitational fields bend the light rays from distant stars when they pass in front of them.

Unfortunately, although surveys of gravitational microlensing have found MACHOs, there’s not enough of them to account for the universe’s missing mass.

So we turn to the other option: WIMPs, or Weakly Interacting Massive Particles. These are a postulated type of subatomic particle that only interacts via gravity and maybe the weak nuclear force.

This means they would have properties similar to neutrinos (i.e. very hard to detect), only heavier (hence the “massive”). And that’s why we haven’t yet been able to create them or detect them on Earth.

However we do have some indications they exist, from observing collisions between galaxies like in the famous Bullet Cluster (pictured).

The so-called Bullet Cluster, formed from the collision of two galaxies, as captured by NASA's Hubble Space Telescope and Chandra X-ray Observatory. Ordinary matter, shown in pink, has been distorted into a bullet shape by the collision, whereas the clusters of dark matter, shown in blue, appear to have passed straight through. This supports the theory that dark matter particles only interact by the force of gravity. (Image from NASA)

In these collisions, we see matter like hot gas piling up in the middle, but the centres of mass of the colliding galaxies – presumably made out of dark matter – pass straight through. This suggests that the dark matter is something that doesn’t interact much at all, like WIMPs.

Lately though, there have been some intriguing signals suggesting we’re getting closer to actually detecting these dark matter particles. Experiments like CRESST and DAMA in Gran Sasso (the same underground lab that detected those pesky neutrinos) have seen hints of WIMPs  colliding with nuclei.

And most recently, the balloon-borne experiment ARCADE has seen a radio signal coming from space that could be the result of WIMPs colliding with each other to produce pairs of electrons and positrons (Fornengo N, Lineros R, Regis M & Taoso M 2011, “A dark matter interpretation for the ARCADE excess?”, arXiv:1108.0569v1).

These are all very small, very early results, so we have to be careful not to jump to conclusions. But when we’re contemplating the universe and trying to come up with a “theory of everything”, we really shouldn’t ignore the other 96% we can’t see.

Recently on the radio

We’ve been a bit quiet recently on the Lost in Science blog. But that doesn’t mean the team hasn’t been busy, oh no!

Here are some links to go with our recent radio broadcasts. Or, you can download the podcasts, for our shows from 3 November 2011 (25:54 min / 12 MB) and 10 November 2011 (28:09 min / 26 MB).

  • Analysis of corporate ownership networks shows that out of 43,060 transnational companies, only 147 of them – mostly banks – control 40% of the wealth. Read more in New Scientist, or see the entire paper in the arXiv database.
  • Protesting about this risks exposure to pepper spray, or Oleoresin Capsicum, which uses the chemical capsaicin ((CH3)2CHCH=CH(CH2)4CONHCH2C6H3-4-(OH)-3-(OCH3)), extracted from chilli peppers, to cause eye and skin irritation. Read about its health effects in Investigative Opthalmology and Visual Science and the North Carolina Medical Journal, or see treatment recommendations from Melbourne’s Royal Children’s Hospital.
  • The Berkeley Earth Surface Temperature study, partly composed of and funded by climate change sceptics, has performed a massive re-analysis of global land temperature records and verified that yes, the world really is warming.
  • Aside from being real, climate change seems to have caused Australian seaweed species to move between 50 and 200 km south, risking the habitat of many other species that depend on them. Read more at ABC Science, or see the paper in Current Biology.
  • In more extinction news, Tasmanian devils are currently threatened by a contagious cancer, which seems to spread due to their genetic similarity. Hope is held for a small, genetically different and mostly disease-free population in the northwest of the state, research into which has won a team of scientists the 2011 Eureka Prize for Environmental Research (also see their paper in Conservation Biology). Although the recent discovery of devils with facial tumour disease in even that remote area has increased concern for this unique species.
  • (A good friend of ours, John Cook of Skeptical Science, was also awarded the 2011 Eureka Prize for Advancement of Climate Change Knowledge. Congratulations John!)
  • Speaking of genetic diversity, research on the Sandy Island mouse has shown that polygamous females produce more viable embryos. See the paper in Ecology Letters, or read more at the University of Western Australia.
  • Finally, to space. Three recent discoveries have shed new light on how solar systems like ours form: there’s a planet called LkCa 15b, 473 light years away, which has been discovered in the process of forming; water seen in the planet-forming disk around the young star TW Hydrae (175 light years away) supports the theory that it collects around grains of dust to make comets, which then deposit the water on planets like Earth; and photos of the asteroid Lutetia, taken by the European Space Agency’s Rosetta probe, suggest that, at around 3.6 billion years old it’s a relic of the early Solar System, and have given clues to its formation.

Have you missed any other shows? Catch up on our old episodes!

Lost in science fiction: Moon

Time for the second review in our week of Lost in Science Fiction, aka science in the movies.

Our first film was not so accurate, but let’s see if we do better with Moon (2009), directed by Duncan Jones, aka Zowie Bowie, and starring almost solely Sam Rockwell.

This is one of those annoying movies with a twist, so I can’t say too much about what happens (although recent research has shown that spoilers can actually make stories better). But suffice to say it’s about a guy (Sam Rockwell) living on the Moon, with only a computerised Kevin Spacey for company.

He’s there to mine for helium-3, an isotope that has one less neutron than the more common helium-4 (which has two neutrons and two protons in its nucleus. Helium-3 still has the two protons and hence the same chemical properties, but lacking a second neutron it has a lower atomic weight).

Helium-3 has been suggested as a possible fuel for nuclear fusion: two helium-3 nuclei can combine to create one helium-4 nucleus and two protons, as well as a whole lot of energy. It’s also used in neutron detectors and to achieve extremely low temperatures in cryogenics.

The trouble is that helium-3 is extremely rare, about 1/10,000th the abundance of helium-4, or around 7.2 parts per trillion in the atmosphere. In fact, most of the helium-3 used on Earth is manufactured.

However, the situation on the Moon is more promising. The lunar regolith, or dirt, may contain up to 50 parts per billion on some parts of the surface. As a result, mining the Moon for helium-3 is a potentially lucrative industry, and it seems to be one of the main reasons the various spacefaring nations are once more interested in lunar exploration.

So, for an interesting depiction of this potential future industry – with a fascinating psychological twist – check out the movie Moon.

Or if you want to try helium-3 mining for yourself, have a go at the Helium-3 Space Game on

Nobel win for Aussie dark energy discovery

Every year, all eyes turn north to see who takes out the Nobel Prizes, the most prestigious scientific awards Sweden has to offer. And in 2011, Australian eyes got to see one of our own, Professor Brian P. Schmidt, receive the Nobel Prize in Physics.

Professor Brian P. Schmidt, of the Australian National University
Professor Brian P. Schmidt, seen here observing the expanding universe (Photo by Belinda Pratten, ANU)

Professor Schmidt shared the prize with Saul Perlmutter, from the Lawrence Berkeley National Laboratory and University of California USA, and Adam G. Riess, Johns Hopkins University and Space Telescope Science Institute USA (actually, in the complex way the Nobel Prize works, Saul Perlmutter got 50% and Brian Schmidt and Adam Reiss each got 25%).

The trio won it for showing that the universe is not only expanding – the first evidence for which was found by Edwin Hubble in 1929 – but its expansion is actually accelerating.

They showed this by observing distant Type Ia supernovae (or supernovas, if you prefer). These incredible explosions occur when white dwarf stars get too big, specifically 1.38 times the mass of our Sun (they usually do this by pulling matter off a nearby star). When they reach this threshold, they collapse under their own weight, and their core compresses further until the pressure sets off a tremendous nuclear fusion reaction that blows the whole thing to smithereens.

What makes these Type Ia supernovae so special is that we know exactly how they work and they’re all exactly the same. So they make great markers that we can look at across the universe. From their brightness we can calculate how far away they are, and from their Doppler redshift we can figure out how fast they’re moving away from us.

The findings that Schmidt, Reiss and Perlmutter announced in 1998 demonstrated that the more distant the supernovas are, the faster they’re moving away from us – and this expansion is getting faster all the time.

The best known explanation for this acceleration is the existence of dark energy, a mysterious, invisible force that permeates all of space. We discussed dark energy – as well as dark matter – and some recent discoveries concerning them a few months ago on Lost in Science (see ‘Help our mass is missing‘). But it’s great to see the original discovery getting such recognition!

Incidentally, according to Wikipedia, Brian P. Schmidt is the 11th Australian to win the Nobel Prize, and our 2nd laureate in physics. Physiology and Medicine is clearly our strong point, with no less than 7 winners, but here’s hoping the physicists are on their way to catching up!

For more on his prize-winning work, see Brian P. Schmidt’s homepage.

Like a diamond in the sky

A sparkling new discovery for an international team of scientists – including Professor Matthew Bailes, Dr Ramesh Bhat and Dr Willem van Straten from Swinburne University in Victoria – was the finding of a planet that’s probably made out of diamond.

Now there are many facets to this discovery, but one that immediately comes to mind is: how do they know it’s made out of diamond?

Well, they’re actually rather clever. The object is orbiting a pulsar – a magnetic, rotating neutron star which emits radio pulses – about 4,000 light years towards the centre of our galaxy. From modulations (or twinkles) in the pulses, it’s possible to tell that there’s something orbiting it every 2 hours and 10 minutes.

From this orbital frequency and an estimate for the mass of the pulsar (all pulsars are between 1.4 and 2 times the mass of our Sun), it’s possible to calculate not only how far away its planet is, but how much its mass is too. We can also calculate its maximum possible radius: anything bigger than the so-called Roche lobe radius will be pulled off by the pulsar.

So we have its mass and its radius, which allows us to calculate its density, from which we can deduce that it must be made of mostly crystalline carbon and oxygen. And of course, crystalline carbon is diamond.

That’d be a diamond with a mass greater than that of Jupiter, around 2.29×1027 kg, or, if you like, 1.14×1031 carats.

For the rock-hard technical details, see Bailes M, Bates SD, Bhalerao V, Bhat NDR, Burgay M, Burke-Spolaor S, D’Amico N, Johnston S, Keith MJ, Kramer M, Kulkarni SR, Levin L, Lyne AG, Milia S, Possenti A, Spitler L, Stappers B & van Straten W, “Transformation of a star into a planet in a millisecond pulsar binary”, Science, published online 25 August 2011, doi:10.1126/science.1208890 [PDF 1.7 MB]

Maybe the multiverse is real in another reality

The idea that there is more than one universe, more than one version of reality, a multiverse if you will, is a very compelling one.

Especially if there are an infinite number of universes: there would have to be others identical – or almost identical – to our own. Somewhere out there could be another you, reading a blog nearly as good as this one.

It’s also an attractive idea for physicists, as it naturally leads to the Anthropic Principle, which is one of the easiest ways to explain why our universe is the way it is – and in particular why it seems to be just right to support intelligent life.

Basically, with an infinite number of possible universes, there has to eventually be one capable of supporting life; and of course, we can only exist in that type of universe.

You can decide yourself whether the Anthropic Principle is a truly satisfying explanation, or just an over-complicated attempt at getting out of explaining anything at all. The question for today is, can we ever know if it’s true?

Normally you’d have to say no: the whole idea of these being separate universes is that they’re, well, completely separate to our own. But there’s one multiverse theory in which each universe exists in a bubble, and if these bubbles were to collide, they’d leave telltale signs.

This from an idea called eternal inflation. Our universe is believed to have gone through a period of extremely rapid inflation early in its life (from around 10−36 to 10−32 seconds after the Big Bang). This inflation stopped when the universe reached a stable state.

But what if there was more space beyond the boundaries of our universe that didn’t ever reach stability? Our universe would be a bubble within this eternally inflating region; and if there’s one bubble, why not more? And what’s to stop them colliding with our own universe?

To see the signs of these collisions you’d have to see the edge of the universe, which of course is a very long way away. But fortunately at the start of the universe it used to be a lot closer, and we can still see the results of this in the leftover heat from the Big Bang, the Cosmic Microwave Background (CMB).

Full sky map of the cosmic microwave background from WMAP, showing the most likely bubble collisions
Full sky map of the cosmic microwave background. The red, green, orange and light blue areas are the areas most likely to be bubble collisions, but even then they're not very likely. The data comes from the Wilkinson Microwave Anisotropy Probe (WMAP).

A team of physicists from University College, London, and elsewhere, have come up with an algorithm for searching the CMB for circular patterns that could be the result of bubble collisions.

So, did they find any? Well, there were a few possibilities (see the image above), but they weren’t really significant enough to separate from random noise. The best they could really do is put an upper limit on how many collisions there are (less than 1.6, at 68% confidence).

Of course, no paper is complete without saying that further data will help their chances of finding something in future. But still: it’s pretty amazing to think that other universes could collide with our own, and we could detect them. You just wouldn’t want to be standing there when it happens.

And also, it’s worth pointing out that even if no other bubble universes ever collided with our own, that doesn’t mean they’re not out there somewhere. After all, the eternally inflating multiverse has to be pretty big.

Read more about this study in the recently published papers, Feeney SM, Johnson MC, Mortlock DJ & Peiris HV 2011, “First observational tests of eternal inflation”, Physical Review Letters 107, 071301 and Feeney SM, Johnson MC, Mortlock DJ & Peiris HV 2011, “First observational tests of eternal inflation: analysis methods and WMAP 7-year results”, Physical Review D 84, 043507.

Or, read a take-down of the media hype around this study – i.e., all the news articles claiming they did find something – on the blog Not Even Wrong.