The Big Bang: a brief guide
last year i noticed a lot of people struggling with the concept of 'theory' in science so i wrote a brief explanation that no one read. in a similar vein a lot of people in debates seem to have no idea about the big bang and i'm sure they wont read this either.
We begin with the fact that the Universe is in one way or another expanding, galaxies can be directly observed to be moving away from one another, those furthest away are moving faster and those closer by less so. we know this from doplershift of absorbtion spectra, specifically called redshift. Because the galaxies are moving apart the light from them becomes 'stretched' or gets longer making light move towards the longer end of the electromagnetic spectrum, the red end (in terms of visable light) objects moving closer together are blueshifted because blue is the shorter wavelength end.
Dopler shift is the effect which makes police sirens get higher as they come towards you (like blue shifted light) and deeper as they go away (like redshifted light).
So how do we know the light is red shifted? by using absorbtion spectra. If you did school physics to a reasonable level you no doubt learned that electrons orbit atoms in set discrete 'shells' (2 in the first then 8 in the second etc). each of these shells has a set energy level and all the electrons in that shell have that energy, if we give those electrons enough energy they can 'move' to a higher energy level (further from the nucleus) and then these electrons can return to lower shells by giving out that energy (as a photon). So electrons in atoms are jumping up and down shells absorbing and emitting energy as photons, which you may know are the particles responsible for light.
as many people know if i look at my green bedroom wall what is happening is that all light except green is being absorbed and the green light is being reflected, thats why i can see it as green. Now you know however that the energy levels in the atoms making up the green paint cause electrons moving down energy levels after being excited up levels by photons from light (sun or electric) to emitt photons around 510nm, but not other wavelengths. every element has a set range of wavelengths that it will absorb and that it will emitt. we can make a spectra of these wavelengths like this one below, this is the emission spectrum of hydrogen. the absorbtion spectrum is the exact oposite because all the wavelengths not emitted are absorbed. (the bright lines are the wavelengths emitted, the black those that are not)
now what has this got to do with the universe expanding? well when we look at the absorbtion spectrums of other galaxies we notice the distinct patterns of well known chemicals as we would expect, however they are redshited. thus we directly measure that the observed galaxy is moving away. this is true of all galaxies, with those furthest away displaying the most redshift.
hopefully you can all see that this is clear evidence for and expanding universe.
as a brief asside because more distant galaxies are moving faster that means that every other galaxy is moving away relative to the observer so every observer appears to be at the centre of the universe.
Now if you are lucky enough to have thoughts you may well be thinking that if all galaxies are moving apart then at some point they were all much closer together and you would be quite logical in your conclusion. A man named Edwin Hubble made lots of measurements of the recesional velocities worked out from their redshift.
eg
z= (L - L0)/L0
where z is the redshift L is the redshifted wavelength and L0 the true wavelength. and when the recesional velocity is small compared to the speed of light the recesional velocity is the redshift times the speed of light.
so if a hydrogen wavelength of 434nm was shifted to 486nm then
z= (486-434)/434 = 0.12. so the galaxy is moving at 0.12 times the speed of light or 3.6x107 m s-1 (81000000 miles per hour)
now if that seems dull thats just to prove we can easily work out how fast a galaxy is moving relative to ours. with all those velocities worked out he too noticed the more distant galaxies moving faster and that the speed was proportional to distance.
velocity is proportional to distance
velocity = hubbles constant x distance
this is known as hubbles law. the value of hubbles constant H is not precise due to the difficulty of accurately measuring distances to far away galaxies accurately but 2.3 x 10-18 s-1 (72 km -s Mpc-1) is fine for what we want to do.
by extropolating backwards we can easily see that galaxies were once much much closer together until all the matter in the universe was in one point, this point is the begging of the universe and only after this point did space or time exist. so when was it?
time is distance divided by speed and speed is as we have seen, distance x hubbles constant. so we get distance / distance x hubbles constant
or 1 / hubbles constant (get your calculator out!!)
1/ 2.3 x 10-18 = 4.3 x 1017 s
or 14 billion years.
and i am soing tired right now that i shall come back and finish this later (read, some other time)
for now you should have learnt
REDSHIFT SHOW GALAXIES ARE MOVING APART
MORE DISTANT GALAXIES ARE MOVING AT HIGHER VELOCITIES THAN LESS DISTANT ONES
FROM REDHSIFT WE CAN WORK OUT A GALAXIES RECESIONAL VELOCITY (the speed its moving away from us)
FROM STUDYING RECESIONAL VELOCITIES WE GET HUBBLES LAW (that velocity is proportianal to distance)
USING HUBBLES LAW WE CAN CALCULATE WHEN ALL MATTER WAS TOGETHER AT THE CENTRE
NEXT UPDATE, COSMIC BACKGROUND RADIATION.
The year is 1964 and two men, Wilson and Penzias are working at the Bell telephone laboratories using a radio telescope to measure the radio emission of the miljy way (thats our galaxy). With the instrument tuned to 4080MHz noise from the galaxy should have been small but they detected an annoying excess of radiation (almost like static). No matter where in the sky they pointed their device or at what time of year the static remained, nor did it change in intensity an intensity which matched a blackbody radiation at 3.5K (-269 Celcius or 3 degrees above absolute zero). In fact the radiation was so constant that it is uniform to one part in one thousand. it would be almost impossible for a local source to create such uniformity.
A Black body is something which both absorbs and emitts perfectly. Wiki describes it thus
(You probably learnt at school that black objects get hotter in sunlight because they absorb and emit the heat more effectively whilst light colours reflect it, well think of a black body as the ultimate tarmac)In physics, a black body is an object that absorbs all light that falls on it. No electromagnetic radiation passes through it and none is reflected. Because no light is reflected or transmitted, the object appears black when it is cold.
If the black body is hot, these properties make it an ideal source of thermal radiation. If a perfect black body at a certain temperature is surrounded by other objects in thermal equilibrium at the same temperature, it will on average emit exactly as much as it absorbs, at every wavelength. Since the absorption is easy to understand—every ray that hits the body is absorbed—the emission is just as easy to understand.
so the intensity of the excess radiation was the sames as that of a blackbody at 3.5 k, the question was, what is it?
At the same time as this a group at princeton were trying to devise a way of finding this radiation, only to hear that Wilson and penzias had done so by mistake. the possibility of the CMB had been put forward in 1948 by George Gamow as a result of the universe beggining in a big bang like fashion.
Why is it that long before it was discovered scientists were already expecting it?
because the universe is expanding outwards we know that it was originally much closer together or to put it better, much denser and because of this much hotter. at half its present size the universe would be 8 times as dense and twice as hot. were it 100th the size it would be the same temperature at which water freezes on earth (273K 0C 32F), imagine space being as warm as a winters day! At one hundred Millionth the current size the temperature would be around 273 million degrees K (491 399 540 degrees Fahrenheit) and the density of matter would be throughout the universe would be the similar to that of air on earth, thats almost incomprihensible compared to the universe now. What is important is that at such temperatures all electrons would have so much energy that none would be bound to hydrogen atoms, the early universe would be full of free electrons and nuclei.
Photons scatter very easily off free electrons and so in the early universe photons interacted with matter in much the same way light does with a fog or a cloud. Basically the early universe was opaque to photons. this multiple scattering would give a 'thermal' or 'blackbody' spectrum of photons throughout the universe. As the universe expands the photons become redshifted, just like we learnt about the glaxies earlier and so the temperature of this thermal spectra would become lower over time.
what is important to remeber is that this was theoretical work showing what we would expect to find if the universe began with a big bang, only after was the CMB actually discovered. A truly fascinating example of finding precisely the proof required for a theory. The team at Princeton confirmed Penzias and Wilsons discovery, stating a blackbody equivalent to 2.7K. Further in 1989 the NASA Cosmic Background Explorer Satelite (COBE) was launched to improve our knowledge on the spectrum and uniformity of the CMB, the results showed the Spectra to be indistinguisable from a blackbody of 2.726K (so accurate that on the following graph the error bars are too small to see)
As a last note i need to point out that the CMB was emitted around 400,000 years after the big bang (some 12-14 billion years ago) at the point when temperatures dropped 'low' enough for atoms to form (i.e. for electrons to bind with nuclei) photons interact very weakly with hydrogen and thus the opaque nature of the universe at that point changed. the popular analogy is that this is very much like a cloud. light interacts strongly with the cloud but weakly with the clear air, photons scatter around inside the opaque cloud until they reach the edge, we can see only the surface of the cloud, the very last point at which the photons interacted with it but not inside the cloud. as such the CMB photons allow us to see to withing 400,000 years of the big bang but not before, just like we cannot see the sky above the cloud.
hopefully you have learned.
THE CMB IS A UNIFORM THERMAL BLACKBODY RADIATION THAT EXISTS THROUGHOUT THE UNIVERSE
IT IS CAUSED BY PHOTONS SCATTERED BY THE MATTER IN THE EARLY UNIVERSE
THE EXISTENCE OF THE CMB WAS PREDICTED 16 YEARS BEFORE ITS DISCOVERY
THE DISCOVERY OF WHICH IS CLEAR PROOF FOR A HOT BIG BANG
I emphasize some of this due to my aims of this thread which is to prove to certain people that the big bang is not mumbo jumbo scientists make up and have no reason to believe.
The Abundance Of Hydrogen and Helium in The Universe
In terms of matter, the universe is almost entirely composed of either Hydrogen or Helium. 73% Hydrogen, 25% Helium and everything else makes up the last 2%. This is a very Strong argument for the big bang as other theories predict much smaller amounts of helium. When the Universe very hot (above 3 x 1010K) photons and neutrons were equally numerous because high temperatures gave sufficent energy to exchange them freely back and forth. when the temperature lowered below this level neutrons stopped exchanging so readily with protons meaning that free neutrons began to decay because they are very unstable, this only takes around 15 minutes so any that were not able to exchange in this time were decaying in to a proton and an electron (and an electron antineutrino)
when temperatures reached 0.9 x 109K then deuterium became stable (deuterium is a hydrogen isotope that has one proton one neutron and one electron, standard hydrogen has no neutron) and so neutrons were taken into neuclei were they are stable and do not decay.
the important issue here is that the time between freen neutron decay and deuterium stability affects how much helium there is in the universe and the amount of helium we observe matches the models of the big bang. another lovely bit of evidence for a hot big bang.
please feel free to ask questions of any nature particularly about the maths aspect.
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...is my daddy!
(My astronomy and cosmology are not as good as they could be.)
Originally Posted by Simetrical
