Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 18/04/2015 02:50:01
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joseph adam asked the Naked Scientists:
As the universe is about 14 billions and the solar system about 4 billion years old, how do we receive signals from the beginning the of the start? what have they been doing for all this time?
Many thanks I really look forward to your podcast (http://www.thenakedscientists.com/HTML/podcasts/)on my MP3 which I listen to whilst walking my daily walk, I need to as I'm 87.
What do you think?
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Not sure if he meant to ask about 'Cosmic background radiation' which is due to big bang and resembles to the signals to which he is referring to, more than the answer which tells us the age of universe. I'm going to state my knowledge on both.
All about cosmic background radiation here: 'http://en.wikipedia.org/wiki/Cosmic_background_radiation'.
How do we detect CBG?
"In 1965 Arno A. Penzias and Robert W. Wilson of Bell Laboratories were testing a sensitive horn antenna which was designed for detecting low levels of microwave radiation. They discovered a low level of microwave background "noise", like the low level of electrical noise which might produce "snow" on a television screen. After unsuccessful attempts to eliminate it, they pointed their antenna to another part of the sky to check whether the "noise" was coming from space, and got the same kind of signal. Being persuaded that the noise was in their instrument, they took other, more sophisticated steps to eliminate the noise, such as cooling their detector to low temperatures.
Finding no explanations for the origin of the noise, they finally concluded that it was indeed coming from space, but that it was the same from all directions. It was a distribution of microwave radiation which matched a blackbody curve for a radiator at about 2.7 Kelvins.
After all their efforts to eliminate the "noise" signal, they found that a group at Princeton had predicted that there would be a residual microwave background radiation left over from the Big Bang and were planning an experiment to try to detect it. Penzias and Wilson were awarded the Nobel Prize in 1978 for their discovery."
Although if he meant to ask how do we know how old the universe is (which has nothing to do with signals) then here's the answer.
"Knowing the current speeds and distances to galaxies, coupled with the rate at which the universe is accelerating, allows us to calculate how long it took for them to reach their current locations. The answer is about 14 billion years (Although a new research shows that the exact age is 13.8 billion years). The second method involves measuring the ages of the oldest star clusters."
How do we measure the age of star clusters?
1) by definition, a main sequence star is one that fuses hydrogen (H) to helium (He) in the core
(the inner 15% by mass) of the star
2) the lifetime of a main sequence star is determined by
Tms = (amount of energy available)/(rate of energy use)
Tms = Mstar fcore fH fmass lost c2/Lstar
where Mstar = the mass of the star
fcore = the fraction of the star's mass that is in the core, i.e., that is converted
from H to He during the main sequence phase = 0.15
fH = the original fraction of the star that is hydrogen fuel = 0.75
fmass lost = fraction of the mass lost in a H --> He fusion reaction = 0.007
Lstar = luminosity of the star = rate at which energy is radiated
3) for main sequence stars, the mass and the luminosity are related:
Lstar = Mstar4
as long as both Lstar and Mstar are in solar units
4) combining the two previous equations, and substituting numerical values for the constants
Tms = 1010 yrs/(Mstar)3
5) note that the larger the mass, the less time a star spends on the main sequence; therefore, in a cluster
(for which all stars were born at the same time, and therefore have the same age), the hotter and brighter stars on the main sequence leave first for the red giant region (where they will fuse H to He in their outer envelope and also fuse He to C in their core)
furthermore, the age of the cluster can easily be found, because it is also the lifetime of the star just now leaving the main sequence... so to find the age of a cluster, use your H-R diagram to find the B-V color index of the star just leaving the main sequence; then use the stellar properties table to convert this to the mass of the star just leaving the main sequence
6) finally, use the relationship in #4 above to determine the main sequence lifetime of that star and, therefore, the age of the cluster
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The rate of the initial expansion, the inflationary period, happened so fast that any available light just could not keep up. That is, if light existed at all during that epoch. It depends upon when the forces separated into strong, weak, electromagnetic and gravitational. It is only at this point that the photon would have the potential to exist as a unique particle.
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Hi Joseph, I guess this question is going to bring us amateurs out of the woodwork bringing lots of waffle. I hope some of it helps. Here's mine. [:)]
Thinking about the CMB some questions must arise, for instance:
1. Where is the radiation coming from?
2. Where is it going?
3. It is travelling at the speed of light, so why had it not passed us long ago?
4. If its origin is in a small spot that must be central to the Universe, does this give a preferred direction to the Universe?
Attempting to answer these questions challenges the lay person's intuitive image of the Big Bang, the expanding Universe and the CMB radiation.
1. Where is the radiation coming from? This implies another question: Where did the Big Bang happen? The answer to this is that it happened everywhere in the Universe. At the first instant, the Big Bang was the Universe; the Universe was the Big Bang. There is no part of the Universe today, nor will there ever be, however long it continues to expand, in which the Big Bang did not occur. The Big Bang was everywhere; so the radiation must be coming from everywhere.
2. Where is it going? If it originated everywhere in the Universe, it follows that it must be going everywhere in the Universe. It is moving at the speed of light from every point in the Universe to every other point in the Universe, without exception.
3. It is travelling at the speed of light, so why had it not passed us long ago? To some extent, this question has already been answered, but if by "us" we mean the point in the Universe at which the Earth is situated, we must accept that it has been passing us ever since the original radiation was able to move freely through the Universe, and it will continue to pass us, and every other point in the Universe, as long as any energy remains in the waves.
4. If its origin is in a small spot that must be central to the Universe, does this give a preferred direction to the Universe? As mentioned earlier, we have to abandon the image of the Big Bang happening at the centre of the Universe, and the radiation emanating from there and moving outward. Viewed from the Earth, the radiation would be seen to be coming from every direction. This does not mean that the Earth is at the centre of the Universe, because, if that image were right, the radiation would appear to be moving away from Earth, and would, therefore, not be visible. Also, the Earth's position is not special. Whatever viewpoint in the Universe an observation might be made from, the radiation would still be observed to be coming from every direction towards that point. Therefore, no preferred direction can be identified by observation of the CMB radiation.
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If you imagine a two-dimensional universe sitting within the surface layer of a three-dimensional balloon, you can then imagine the balloon initially being very small with the two-dimensional space being filled with light which is going in all directions (fully contained within that two-dimensional surface). This light will just keep going round and round forever. If you expand the balloon, the light will be stretched and will reduce in frequency, thereby turning it from light into radio waves. These waves will keep on going round and round within the two-dimensional layer though as before, so a two-dimensional planet sitting anywhere in this space will always be bathed in this radiation, and over time the frequency of the radiation will continue to fall as the balloon expands. (Some of the light/microwaves will be absorbed by matter, so it isn't all going to last forever, but there is so much empty space and so little matter that only a tiny amount is being lost.)
Now all you have to do is imagine the same thing happening again but with three space dimensions tied up in the surface layer of the balloon instead of two.
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One advantage of dog walking is that I have about an hour each day when, apart from interaction with other dogs and their walkers, I can do some thinking. The following is a record of some of my thoughts arising from this thread. I would really appreciate some comments.
What do we non-scientists visualise when we think about the expansion of the Universe? Certainly, a common image is that of an expanding sphere, like an inflating balloon. This, of course, is not to be confused with the “balloon analogy” in which the Universe is represented by the two-dimensional surface, rather than by the whole balloon.
Let’s think briefly about this visualisation. Without in any way suggesting that the Universe actually has a physical boundary, the mental image will almost certainly have one. You, of course, will be at the centre of this sphere, and will see the rest of the Universe moving away from you. The most up to date figure for the expansion rate of the Universe is “67.3 kilometers per second per megaparsec”. A megaparsec is defined as a distance equal to 3.26 million light years.
Just to make the arithmetic easy, let’s say our imaginary sphere has a radius of one thousand megaparsecs. Obviously, this is much too small, but it is just a simple thought experiment. This means that the imaginary boundary will be separating from you at 67,300 kilometres per second. The galaxies are being carried along with the expanding space, so any celestial body that is within the last 67,300 km. on this side of the boundary now, will be outside the present boundary position in one second’s time. It will be in newly created space; space that was not there a second ago.
Think a little more about this, though. An observer, say on a planet that has, from your perspective, just passed into the newly created space, will perceive himself as being at the centre of a spherical universe, and will think that you have just passed into newly created space.
This tells us a couple of things: (1) If the Universe is homogeneous, and in order to do any meaningful cosmology we must assume it is, we cannot define a physical boundary. (2) We cannot define any part of the Universe; rather than any other part; as being the newly created part. This must mean that every part of the Universe is expanding: new space is coming into existence everywhere in the Universe, all the time. No part is newer than any other part.