Tag: universal expansion

The Universe is still expanding. As expected.

By Phil Plait | October 4, 2012 10:31 am

The Universe is getting bigger!

But then, we knew this. We’ve known it for a long time! The reason you know Edwin Hubble’s name at all is because in the 1920s he was critical in figuring out the Universe was expanding. He and many other people did this by looking at a specific kind of star, called Cepheid variables. These stars literally pulsate, getting brighter and dimmer on a regular schedule. As it happens, how much they change in brightness depends on their actual brightness… and that means if you measure how much they change, and how bright they appear in our sky, you can figure out how far away they are. And if they are in other galaxies, then you can tell how far away those galaxies are.

Boom! You can measure the size of the Universe. And more.

Using this method (which I explain in more detail in an earlier post, if you want details), they figured out the Universe was expanding – the farther away a galaxy is, the faster it appears to recede away from us. This is what led to the Big Bang model of the Universe, and essentially all of modern cosmology – the study of the origin, evolution, and properties of the Universe as a whole.

Over the decades, that rate of expansion – called the Hubble Constant – has been measured many different ways. Using Cepheid variables is still a foundation of the work, though, and a new study just released by astronomers using the Spitzer Space Telescope show that the rate of expansion is 74.3 +/- 2.1 kilometers per second per megaparsec. What this means is that a galaxy one megaparsec away (that is, 3.26 million light years) will be moving away from us at 74.3 km/sec. If you double the distance to 2 megaparsecs, a galaxy would be moving away at twice that speed, or 148.6 km/sec.

This study is pretty neat. Spitzer observes in the infrared, which can pass right through interstellar dust. That dust is like a fog, obscuring the visible light from stuff behind it, and it really messes with measuring brightnesses. That has plagued Cepheid studies for years, but Spitzer simply steps around that problem! So this measurement appears to be pretty accurate, more so because they calibrated it using Cepheids in our own galaxy (and one nearby one), and combined it with results from other observatories like WMAP, which can measure other properties of the Universe as well. By doing all this, they’ve produced a very accurate measurement of the Hubble Constant.

I want to be clear here: this new study is more accurate than previous ones, and much more accurate than one done a few years ago using Hubble. However, a study done just last year got the expansion rate to an accuracy of about 3.3%, and that used a combination of Cepheids and Type Ia supernovae – a star that explodes with a measurable and predictable brightness. This new study has an accuracy of just under 3% – an improvement for sure, though not a huge one over last year’s.

Still, this is very cool. That last study got a rate of 73.8 +/- 2.4 km/sec/megaparsec, so they both agree closely within their error margin. In fact, they’re statistically identical (and agree with quite a few other measurements made in the past, too). That’s good! It means we’re really nailing this number down, and that’s further evidence we really do have a pretty good basic understanding of how the Universe is expanding.

There’s still a lot to figure out in cosmology; we don’t know what dark matter is, and about dark energy we know even less. But it’s good that people are looking into other ways to measure basic properties of the Universe. The more we know them, the less we have to worry about them. And it shows that our overall model holds up. The Universe had a beginning, 13.7 billion years ago. It was small then, but it’s been expanding ever since, and in fact is expanding faster every day. We are a small part of it – in fact, our matter is a small part of all the matter (that dark stuff dominates by a lot) and even that is a small part of the stuff that makes up the Universe (dark energy wins that round).

And as amazing as all that is, I am even more astonished by the fact that we can know any of this at all! The Universe obeys rules, and by doing so reveals those rules to us. We just have to be smart enough to investigate them and learn about them.

And we are that smart.

Image credits: Image credit: NASA/JPL-Caltech; CBS


Related Posts:

- The Universe is expanding at 73.8 +/- 2.4 km/sec/megaparsec! So there.
- The Universe is expanding at 74.2 km/sec/Mpc
- Wait a sec. How big is the Universe again?
- The Universe is 13.73 +/- .12 billion years old!

CATEGORIZED UNDER: Astronomy

An ultradeep image that's *full* of galaxies!

By Phil Plait | March 23, 2012 6:00 am

What happens when you take a monster 4.1 meter telescope in the southern hemisphere and point it at the same patch of sky for 55 hours?

This. Oh my, this:

[Click to embiggen.]

OK, I know. At first glance it doesn’t look like much, does it? Just a field of stars. However, here’s the important bit: I had to take the somewhat larger original image and reduce it in size to fit my 610-pixel-wide blog. So how much bigger is the original?

It’s 17,000 x 11,000 pixels! If you happen to be sitting on a T1 line, then you can grab this massive 250 Mb file. And I surely suggest you do.

Because yeah, the brightest objects you see in this are stars. Probably a few hundred of them. But you have to look at the bigger image ! Why? Because what’s amazing, truly jaw-dropping and incredible is this:

There are over 200,000 galaxies filling this image!

Ye. Gads.

Here’s a zoom of the image, centered on what looked to me to be one of the biggest galaxies in the frame, a nice edge-on spiral.

With the exception of a handful of blue-looking stars, everything in this zoom is a galaxy, probably billions of light years away. Those tiny red dots are galaxies so far away they crush our minds to dust: we’re seeing them with light that left them shortly after the Universe itself formed.

This light is ancient. And it came a long, long way.

By the way, that picture of the spiral there is not even at full resolution! Just to give you an idea, I cropped out just that galaxy in the full-res image and inset it here. If you want to find it in the full frame, it’s about one-third of the way in from the left, and one-third of the way down from the top. Happy hunting.

[Edited to add: I forgot to add that this galaxy is warped! See how the disk flares up on the left and down on the right, just a bit? This is very common in disk galaxies, and our own Milky Way does it too (see #9 at that link). It's usually caused when a nearby galaxy's gravity torques on the stars in the disk.]

These images were taken with VISTA, the European Southern Observatory’s Visible and Infrared Survey Telescope for Astronomy (VISTA), a 4.1 meter telescope in Chile. This huge image is actually composed of 6000 separate images, and is the single deepest infrared picture of the sky ever taken with this field of view. Hubble can get deeper, for example, but sees a much, much smaller part of the sky.

Read More

CATEGORIZED UNDER: Astronomy, Pretty pictures

The Universe is expanding at 73.8 +/- 2.4 km/sec/megaparsec! So there.

By Phil Plait | March 22, 2011 7:00 am

In 1998, two teams of astronomers independently reported amazing and bizarre news: the Universal expansion known for decades was not slowing down as expected, but was speeding up. Something was accelerating the Universe.

Since then, the existence of this something was fiercely debated, but time after time it fought with and overcame objections. Almost all professional astronomers now accept it’s real, but we still don’t know what the heck is causing it. So scientists keep going back to the telescopes and try to figure it out.

[Click to galactinate, or grab the cosmic 3500 x 4000 pixel browser bruiser.]

This gorgeous image is of the nearby spiral galaxy NGC 5584, where of course "nearby" to an astronomer means 72 million light years. This galaxy is loaded with a specific type of variable star — called Cepheids — which are very important: the way they change their brightness depends on how luminous they are. Measure the change, and you measure the luminosity, and if you measure how bright they appear in the sky you get their distance. It’s a bit like judging how far away a car is by gauging how bright its headlights are. Except in this case astronomers use Hubble instead of their eyes. It’s a tad more accurate.

It so happens that in 2007, NGC 5584 was the host of a Type Ia supernova, the Golden Standard of distance indicators. These are so bright they can be seen clear across the Universe! By knowing the distance to the one in NGC 5584, we can then use that to get the distances to supernovae much, much farther away.

It’s a bootstrappy way of measuring the cosmic distance scale.

But it appears to work. Read More

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