Chief of the Laboratory for Exoplanets and Stellar Astrophysics at NASA Goddard Space Flight Center
Gazing at the Universe from Different Perspectives:
· 1 light year: the distance light travels in a year is about 10,000,000,000,000 km away
· Nearest star in our own galaxy is 38,000,000,000,000 km away (4.2 light years)
· Nearest neighbor spiral galaxy Andromeda is 2 million light years away
· We see galaxies at distances of several billion light years away, looking as they did that long ago when the light left the galaxy.
There are about 100 billion galaxies, and at least 100 billion stars in each one, meaning there are roughly 10,000 billion billion (10,000,000,000,000,000,000,000) stars in the universe! ...
[image at 20:05] The green circles represent galaxies identified as the most distant, the most what we called “red-shifted”. Edwin Hubble found out decades ago that the universe is expanding, which means that as space expands, the galaxies are caught up in that expansion of space, and the most distant of galaxies from us appeared to be moving away from us faster than the ones that are closer to us because of this universal expansion of space. ...
Why do we care? Well, we live on carbon and oxygen; we depend on star factories to make them. Very, very interesting process.
[image 24:35] We can see progression throughout the history of the universe. So this is a map, funny looking map, but it’s a map of the background radiation that we see in every direction in the sky. So, this is the full sky, looking at every direction but projected onto a plain. What we’re seeing is the leftover radiation that was predicted would be there if our universe had a very energetic and precise beginning point in time.
The universe is a very active place. Here are two galaxies. I told you that galaxies are carried along the expansion of space, and so for the most part they are moving away from each other.
But there are times when galaxies are so close to each other that their gravitational attraction for each other can overcome that otherwise tendency to move apart from each other. ...
There’s a lot of things we don’t quite understand yet. One of them is the whole idea of MATTER THAT WE CANNOT SEE. It turns out that when you look carefully at the rotation of stars in the galaxies, you can tell that there’s a lot of matter that we don’t see that must be there.
Stars are basically orbiting the center of mass of galaxies.
It’s a simple calculation for those of you who have taken physics or astronomy courses, to calculate the velocity of something in orbit by just knowing the mass of the object and the mass within its orbit. It turns out that there must be much more mass in galaxies than is evident through our observations to account for the rotation speed of the material in the galaxies. We called this DARK MATTER because we don’t know what it is. And it’s not quite clear even if we could guess things that we might be able to see, like planets or small things, there’s still not enough of it to account for what it is.
So, we don’t know what it is. But interestingly enough, we can still can make maps of it, even though it’s dark and we can’t see it. How is that? Well, you can see its effects on things. This is a very clear picture [36:50] taken of a cluster of galaxies. Imagine how much mass. If you have a cluster with that much mass, it can act as something called a “gravitational lens”. Einstein and others predicted that something with great mass could even bend light that comes near by it.
So, we now know that clusters of galaxies are massive enough to bend light coming from background galaxies, enough that we can detect that “gravitational lensing” as we call it. And we see distorted shapes of galaxies in the background being distorted as they come near these clusters.
[same image at 37:40] Well, this funny looking picture is a combination of just a regular picture of cluster of galaxies and the blue shades are put in to represent those regions where the background galaxies in the picture have the most distorted shapes. That tells you where the foreground cluster has the most dark matter. It’s a clever way of mapping in this one system where the most dark matter is by mapping out its effect on the light it comes through that region, even though we can see the matter itself.
So, there’s a lot of study on dark matter even though we don’t know what it is. But it seems to act like normal matter in many ways; we just need to understand what it is.
[image 38:30] There’s something even stranger going on. The last decade, we have realized that our universe, the expanding universe, is not only expanding but it is accelerating.
The big question when I was in graduate school was whether gravity would pull the expansion back together again into a big crunch, or whether that expansion would accelerate, decelerate (sorry) forever but slowing down. Because gravity was the only thing we knew that work on such a large scale, and that would only work to slow the expansion down.
Now, we found, by looking at very distant galaxies carefully and comparing them to the expansion rates of nearer galaxies, that the universe expansion for much of its history has been accelerating. And we don’t know what causes the acceleration on such a large scale.
Channel News Asia | 4 March 2016