Measuring cosmic distances

When you look up at the night sky there’s no way you can immediately tell which stars are closest. Are they very bright and far away or are they very dim and quite close?

This is a problem astronomers have also struggled with in the past and if we want to understand the universe properly we need to know how to measure cosmic distances accurately. I’ll explain some of the ways astronomers do that.

Let’s say you send three friends out into the woods at night with torches with differing brightnesses and they all stop at different distances away and shine their torches at you. If you don’t know who’s ended up where and which torch is which, you can’t figure out who’s closest and who’s furthest away.

All you’re seeing is the apparent brightness of the torches, which is how bright the torch looks to you.

In order to start measuring the distances to these torches it would be useful to know the intrinsic brightness of the torches, which is how bright each torch actually is. If all three torches were the same distance away, the one with the highest intrinsic brightness would also be the one with the highest apparent brightness.

But, as it stands, you have three torches at an unknown distance and you don’t know the intrinsic brightness of any of them. So let’s say these torches have another property such that they pulse in relation to how bright they are: the dimmest torch pulses three times a second, the middle torch pulses two times a second and the brightest torch pulses once a second.

You will now know the intrinsic brightness of each torch based on how fast it pulses. You can compare that to the apparent brightness you see from the torches and figure out how far away each torch is.

There is a class of star called a cepheid variable that pulses in relation to its brightness, just like your torches do. When astronomers find something like this, where the “known physics” of an object helps them determine its distance, they call them standard candles.

Cepheid variable stars.

If you find a cepheid variable in a distant galaxy you have a rough idea how far away that galaxy is. But, on the cosmic scale, cepheids aren’t all that bright so finding them in really distant galaxies is extremely difficult. We need something brighter.

Something that’s very bright is the explosion of a Type 1a Supernova. These occur when two stars of certain masses orbiting each other get too close together. This triggers a nuclear fusion explosion that is very bright indeed, often outshining all the stars in an entire galaxy.

Because we know the masses stars must be in order to trigger such an explosion, and because we understand the physics of such a nuclear fusion explosion, we know it’ll reach a certain peak (intrinsic) brightness. And because we know all this we can use Type 1a Supernovas as standard candles too.

In fact there are half a dozen or so of these cosmic events we can use as standard candles.

I have made it sound (relatively) easy so far but it isn’t. For starters not all astronomers agree with the accuracy of particular standard candles. Then there’s the issue of the intrinsic brightness we see — light from these cosmic events has to travel a long way to reach us and in the process it may be dimmed by things like the inter-stellar dust it passes through.

These things need to be taken into consideration and accurate measurements of cosmic distances need to use as many standard candles as possible.

So where am I going with this, because standard candles are unlikely to help you find your way to the post office? They are however important when it comes to determining how fast the universe is expanding.

Cosmologists recently got together to discuss the expansion of the universe and it seems things may not be quite as we thought.

The standard model of cosmology is called the Lambda-CDM model (ΛCDM). The ‘CDM’ stands for Cold Dark Matter.

From about the 1970s onwards astronomers began to notice that what we see in the universe shouldn’t really be happening. Specifically, they noticed that the rotation of galaxies is all wrong if we base it on the matter we can see in stars, planets and dust clouds. There simply wasn’t enough matter by a long way. In fact there was only about 15% of the matter we needed to provide the observed rotation.

What we needed was something to account for the other 85% of matter. It couldn’t be seen and it didn’t seem to interact with anything except gravity, but it had to be there or galaxies would not rotate the way they do. So they called this ‘stuff’ Dark Matter and they’re still trying to figure out what it actually is.

Lambda (Λ) is something they often call the cosmological constant, which represents the energy density of space. This drives the expansion of space. If it was zero then space wouldn’t be expanding, but it is expanding and it’s expanding to such a degree that there must be a fair bit of it.

This energy density is also called the vacuum energy and quantum mechanics has something to say about that. “Nature abhors a vacuum,” said Aristotle, and he may have been correct. Quantum fields permeate space and they’re never entirely silent. They fluctuate on an ongoing basis and this gives rise to energy.

There is much wrong with the theory behind the quantum vacuum and it is often described as "the worst theoretical prediction in the history of physics”. This is because the theory puts the value of the vacuum energy 120 orders of magnitude greater than we observe.

But, to get back to the point, there is some vacuum energy that’s driving the expansion of the universe. This is what cosmologists sometimes call Dark Energy.

Together, Dark Matter and Dark Energy constitute over 95% of the universe and we can’t see any of it. Everything you’ve ever seen or encountered is part of the 5% that isn’t Dark Matter or Dark Energy.

This Lambda-CDM model makes certain predictions about how fast the universe is expanding and that stands at roughly 67km/sec per megaparsec.

A parsec is a cosmic (angular) measurement astronomers use to measure great distances. It’s the equivalent of 3.26 light years or 19 trillion miles. A megaparsec is a million of those, so it’s a long way in everyday terms but perhaps not such a long way in cosmic terms. Andromeda, the nearest galaxy to us, is approximately three-quarters of a megaparsec away from us.

The Hubble constant over time.
The Hubble constant over time.

So for every megaparsec away from us, objects are receding at an additional 67km/sec. This figure is known as the Hubble Constant. It's the same sort of figure the Lambda-CDM theory wants and that’s what it had based on some of the early measurements of the expansion of the universe.

However, recent measurements — using more of the standard candles I mentioned above — seem to show the expansion rate is close to 74km/sec per megaparsec, and they show this to a high degree of certainty.

It may not seem like much of a difference but it’s enough to put the current Lambda-CDM model of cosmology at risk, hence the conference of cosmologists and astronomers. Either we need to rethink Lambda-CDM or there’s something very wrong with our observations, or possibly with the objects we use as standard candles.

I’ll grant you this may not ruin your own particular day.