For what you would like to see on Exploring the Cosmos?
Anonymous asked: What do you think of Lawrence Krauss's "A Universe From Nothing"
The lecture is absolutely wonderful. I’ve watched it several times and I love it each and every single time. The book is also incredible and I highly recommend both of them to anyone interested in the origin and evolution of the Universe.
As a side note, Lawrence Krauss will be in Toronto in the very near future giving a lecture on The Physics of Star Trek. I’m tempted to go even though I’ve never seen anything of Star Trek just because Lawrence Krauss is so badass.
Anonymous asked: What is the Cosmos?
The term “universe” was traditionally used to encompass everything that exists. If if exists, it is part of the universe. Now, there are multiverse theories which suggest that our Universe isn’t the only universe. It has now become widely popular to refer to “everything” as the Cosmos and our particular universe the Universe. It seems like silly semantics, but we no longer discuss only our Universe because many people believe there exists something outside our Universe. Cosmos is the next best thing; it encompasses all universes. Our Universe is part of the Cosmos.
Anonymous asked: can you look through a strong telescope back into time ?
Looking out into space is looking back into time; that’s the beauty of it. You don’t need a big telescope to do so. In fact, you don’t even need a telescope at all to look back into time.
Consider the fact that light travels at a finite speed. The Sun is 1 AU (one astronomical unit) away from the Earth. 1 AU = 150 million km. It takes light leaving the surface of the Sun approximately 8 minutes to reach the Earth. This means that if you were to look at the Sun (which you should never do), what you’re actually seeing is 8 minutes into the past! Seeing something like the Andromeda galaxy is looking even farther back into time because it is even further away from us. The Andromeda galaxy (our neighbour), is 2.5 million light years away from us, which means that it takes light 2.5 million years to get to us and thus when you look at Andromeda, you are actually looking 2.5 million years into the past. The further out into space you look, the further back in time you are seeing. This is simply because the light you are seeing travels at a finite speed and by the time the light reaches us, time has passed since the photons left. The furthest back in time we can see is looking at the CMB radiation which was released when the Universe was only 400,000 years old. This light is detected as radio waves and sadly, we cannot see any further back into time (for the same reason we can’t see inside the Sun.) Otherwise, in principle, we would be able to see the beginning of the Universe! Can you imagine?!
Anonymous asked: Why does the expansion of the universe increase the wavelength of photons, but not their size?
Simply because photons don’t have a physical size! It’s strange, isn’t it? Photons have linear and angular (spin) momentum but they don’t even have a physical size. They are dimensionless particles. Similarly with electrons. The characteristic of such particles that does have a size, though, is their wavelength which does get stretched.
The smallest structures that are affected by the expansion rate of the Universe are superclusters of galaxies. Our own supercluster (the Virgo supercluster) is marginally bound by gravitational attraction. Everything within our supercluster of galaxies is being held together by gravitational attraction against the expansion of the Universe.
Anonymous asked: In your post "post/21800557534/if-the-universe-is-expanding-does-that-mean-that" you stated that "Astronomers commonly probe the depths of space that reveal redshifts larger than one (z>1) which suggests that these observed objects are receding at greater speeds than that of light." If, for example, distant stars are receding faster than the speed of light, then how do the photons they emit reach us? Or are astronomers probing the depths of space without the aid of photons?
Very good question! This startled me at first too, and I’m glad you picked up on this. It was not a mistake; for example, the Cosmic Microwave Background radiation had a temperature of approximately 3,000K when it was emitted and measurements suggest it has a current temperature of ~2.725K. For simplicity, let’s say the current CMB temperature is 3K. The CMB, then, had been stretched by a factor of 1,000 and thus had been redshifted by about 1,000. To merely observe the CMB radiation which happened when the universe was about 400,000 years old is to probe so far into space that the redshift is as high as 1,000.
Now, this is where your confusion arises; does this mean that objects as far as the CMB are moving 1,000 times the speed of light? Is the CMB that we detect moving at 1,000 times the speed of light? Well, no. There’s two different types of redshifts that can be measured: the real motion of objects (which is given by the Doppler Shift of the motion of objects in the radial direction which we can measure from their emission spectra) and then there’s measuring the expansion rate of the Universe itself. At low redshifts, the measurements are significantly due to the real motion of objects, but at high enough redshifts, especially redshifts as high as 1,000, that is certainly a measurement of the expansion rate of the Universe itself.
Though objects such as galaxies cannot move faster than light, the expansion rate of the Universe is an exception. In my first year of my undergraduate studies, what I’m about to tell you was by far the coolest thing I learned: on larger scales, nothing is actually moving. Distant galaxies and their spectra suggest that they are moving away from us (that’s what their redshifts tell us), but in reality, it’s actually the expansion of the Universe that is causing their redshifts. It is the space BETWEEN GALAXIES that is expanding, and while photons are traveling within this space, these photons get stretched by the expansion of the Universe itself which makes them look red. Thus, though we talk about recessional velocities of distant galaxies, it’s actually the expansion rate of the Universe that we’re measuring, which is allowed to exceed the speed of light. So, the CMB isn’t receding at 1,000 times the speed of light… the Universe at that point is expanding at 1,000 times the speed of light.
Thank you for this wonderful question. I always love explaining this to people.