megaparsecs Archives

October 12, 2015February 23, 2017 by Fraser Cain

How Are Galaxies Moving Away Faster Than Light?

So, how can galaxies be traveling faster than the speed of light when nothing can travel faster than light?

I’m a little world of contradictions. “Not even light itself can escape a black hole”, and then, “black holes and they are the brightest objects in the Universe”. I’ve also said “nothing can travel faster than the speed of light”. And then I’ll say something like, “ galaxies are moving away from us faster than the speed of light.” There’s more than a few items on this list, and it’s confusing at best. Thanks Universe!

So, how can galaxies be traveling faster than the speed of light when nothing can travel faster than light? Warp speed galaxies come up when I talk about the expansion of the Universe. Perhaps it’s dark energy acceleration, or the earliest inflationary period of the Universe when EVERYTHING expanded faster than the speed of light.

Imagine our expanding Universe. It’s not an explosion from a specific place, with galaxies hurtling out like cosmic jetsam. It’s an expansion of space. There’s no center, and the Universe isn’t expanding into anything.

I’d suggested that this is a terribly oversimplified model for our Universe expanding. Unfortunately, it’s also terribly convenient. I can steal it from my children whenever I want.

Imagine you’re this node here, and as the toy expands, you see all these other nodes moving away from you. And if you were to move to any other node, you’d see all the other nodes moving away from you.

Here’s the interesting part, these nodes over here, twice as far away as the closer ones, appear to move more quickly away from you. The further out the node is, the faster it appears to be moving away from you.

This is our freaky friend, the Hubble Constant, the idea that for every megaparsec of distance between us and a distant galaxy, the speed separating them increases by about 71 kilometers per second.

Galaxies separated by 2 parsecs will increase their speed by 142 kilometers every second. If you run the mathatron, once you get out to 4,200 megaparsecs away, two galaxies will see each other traveling away faster than the speed of light. How big Is that, is it larger than the Universe?

The first light ever, the cosmic microwave background radiation, is 46 billion light-years away from us in all directions. I did the math and 4,200 megaparsecs is a little over 13.7 billion light-years.There’s mountains of room for objects to be more than 4,200 megaparsecs away from each other. Thanks Universe?!?

Most of the Universe we can see is already racing away at faster than the speed of light. So how it’s possible to see the light from any galaxies moving faster than the speed of light. How can we even see the Cosmic Microwave Background Radiation? Thanks Universe.

WMAP data of the Cosmic Microwave Background. Credit: NASA

Light emitted by the galaxies is moving towards us, while the galaxy itself is traveling away from us, so the photons emitted by all the stars can still reach us. These wavelengths of light get all stretched out, and duckslide further into the red end of the spectrum, off to infrared, microwave, and even radio waves. Given time, the photons will be stretched so far that we won’t be able to detect the galaxy at all.

In the distant future, all galaxies and radiation we see today will have faded away to be completely undetectable. Future astronomers will have no idea that there was ever a Big Bang, or that there are other galaxies outside the Milky Way. Thanks Universe.

I stand with Einstein when I say that nothing can move faster than light through space, but objects embedded in space can appear to expand faster than the speed of light depending on your perspective.

What aspects about cosmology still give you headaches? Give us some ideas for topics in the comments below.

February 9, 2015December 23, 2015 by Brian Koberlein

How Fast is the Universe Expanding?

Artists illustration of the expansion of the Universe (Credit: NASA, Goddard Space Flight Center)

The Universe is expanding, but how quickly is it expanding? How far away is everything getting from everything else? And how do we know any of this anyway?

When astronomers talk about the expansion of the Universe, they usually express it in terms of the Hubble parameter. First introduced by Edwin Hubble when he demonstrated that more distant galaxies are moving away from us faster than closer ones.The best measurements for this parameter gives a value of about 68 km/s per megaparsec.

Let’s recap. Hubble. Universe. Galaxies. Leaving. Further means faster. And then I said something that sounded like “blah blah Lando blah blah Kessel Run 68 km/s per megaparsec”. Which translates to if you have a galaxy 1 megaparsec away, that’s 3.3 million light years for those of you who haven’t seen Star Wars, it would be expanding away from us at a speed of 68 km/s. So, 1 megaparsec in distance means it’s racing away at 68 km/s.

This is all because space is expanding everywhere in all places, and as a result distant galaxies appear to be expanding away from us faster than closer ones. There’s just more “space” to expand between us and them in the first place. Even better, our Universe was much more dense in the past, as a result the Hubble parameter hasn’t always had the same value.

There are two things affecting the Hubble parameter: dark energy, working to drive the Universe outwards, and matter, dark and regular flavor trying to hold it together. Pro tip: The matter side of this fight is currently losing.

Expansion of the Universe. Image credit: Eugenio Bianchi, Carlo Rovelli & Rocky Kolb.

Earlier in the Universe, when the Hubble parameter was smaller, matter had a stronger influence due to its higher overall density. Today dark energy is dominant, thus the Hubble parameter is larger, and this is why we talk about the Universe not only expanding but accelerating.

Our cosmos expands at about the rate at which space is expanding, and the speed at which objects expand away from us depends upon their distance. If you go far enough out, there is a distance at which objects are speeding away from us faster than the speed of light. As a result, it’s suspected that receding galaxies will cross a type of cosmological event horizon, where any evidence of their existence, not even light, would ever be able to reach us, no matter how far into the future you went.

What do you think? Is there anything out there past that cosmological event horizon line waiting to surprise us?

December 8, 2014March 1, 2017 by Fraser Cain

How Do We Measure Distance in the Universe?

This star is X light-years away, that galaxy is X million light-years away. That beginning the Universe is X billion light-years away. But how do astronomers know?

I’m perpetually in a state where I’m talking about objects which are unimaginably far away. It’s pretty much impossible to imagine how huge some our Universe is. Our brains can comprehend the distances around us, sort of, especially when we’ve got a pile of tools to help. We can measure our height with a tape measure, or the distance along the ground using an odometer. We can get a feel for how far away 100 kilometers is because we can drive it in a pretty short period of time.

But space is really big, and for most of us, our brains can’t comprehend the full awesomeness of the cosmos, let alone measure it. So how do astronomers figure out how far away everything is? How do they know how far away planets, stars, galaxies, and even the edge of the observable Universe is? Assuming it’s all trickery? You’re bang on.

Astronomers have a bag of remarkably clever tricks and techniques to measure distance in the Universe. For them, different distances require a different methodologies. Up close, they use trigonometry, using differences in angles to puzzle out distances. They also use a variety of standard candles, those are bright objects that generate a consistent amount of light, so you can tell how far away they are. At the furthest distances, astronomers use expansion of space itself to detect distances.

Fortunately, each of these methods overlap. So you can use trigonometry to test out the closest standard candles. And you can use the most distant standard candles to verify the biggest tools. Around our Solar System, and in our neighborhood of the galaxy, astronomers use trigonometry to discover the distance to objects.

They measure the location of a star in the sky at one point of the year, and then measure again 6 months later when the Earth is on the opposite side of the Solar System. The star will have moved a tiny amount in the sky, known as parallax. Because we know the distance from one side of the Earth’s orbit to the other, we can calculate the angles, and compute the distance to the star.

I’m sure you can spot the flaw, this method falls apart when the distance is so great that the star doesn’t appear to move at all. Fortunately, astronomers shift to a different method, observing a standard candle known as a Cepheid variable. These Cepheids are special stars that dim and brighten in a known pattern. If you can measure how quickly a Cepheid pulses, you can calculate its true luminosity, and therefore its distance.

Hubble Frontier Fields observing programme, which is using the magnifying power of enormous galaxy clusters to peer deep into the distant Universe.. Credit: NASA. — Hubble Frontier Fields observing programme, which is using the magnifying power of enormous galaxy clusters to peer deep into the distant Universe. Credit: NASA.

Cepheids let you measure distances to nearby galaxies. Out beyond a few dozen megaparsecs, you need another tool: supernovae. In a very special type of binary star system, one star dies and becomes a white dwarf, while the other star lives on. The white dwarf begins to feed material off the partner star until it hits exactly 1.4 times the mass of the Sun. At this point, it detonates as a Type 1A supernova, generating an explosion that can be seen halfway across the Universe. Because these stars always explode with exactly the same amount of material, we can detect how far away they are, and therefore their absolute brightness.

At the greatest scales, astronomers use the Hubble Constant. This is the discovery by Edwin Hubble that the Universe is expanding in all directions. The further you look, the faster galaxies are speeding away from us. By measuring the redshift of light from a galaxy, you can tell how fast it’s moving away from us, and thus its approximate distance. At the very end of this scale is the Cosmic Microwave Background Radiation, the edge of the observable Universe, and the limit of how far we can see.

Astronomers are always looking for new types of standard candles, and have discovered all kinds of clever ways to measure distance. They measure the clustering of galaxies, beams of microwave radiation from stars, and the surface of red giant stars – all in the hopes of verifying the cosmic distance ladder. Measuring distance has been one of the toughest problems for astronomers to crack and their solutions have been absolutely ingenious. Thanks to them, we can have a sense of scale for the cosmos around us.

What concept in astronomy do you have the hardest time holding in your brain? Tell us, in the comments below.

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