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Brief perspective on notion of the big bang

  1. Jul 21, 2003 #1

    marcus

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    The notion of the big bang is familiar to everybody at PF. The idea has dominated how people think about cosmic history maybe since around 1965 with the detection of the CMB.

    Widely shared myths or images about beginnings are important parts of culture. If anyone has a link to some online perspective about the big bang please post it. I want to list here what I think are the essentials in the story of this idea----which for me and probably for most of us is not just a myth but something we believe. Every popular idea, true or not, has a history. Please correct any mistakes.

    BTW I think that in the future the Big Bang concept will change
    for two main reasons: the neutrino background from the first second of expansion will be observed (discussed in a brief survey paper by Ringwald http://arxiv.org/abs/hep-ph/0301157) and quantizing General Relativity will eliminate the glitch at time zero (discussed by Ashtekar in understandable non-technical terms http://arxiv.org/abs/math-ph/0202008).

    The expectation that this central idea or story will change is the reason I want to review the history of it.


    As I see it the important events are:

    1916 Einstein comes up with GR equation which seems to require for stability a (possibly very gradual) expansion or contraction. This bothers him so he puts an extra term in it as a kludge to get a steady state. This kludge does not actually produce a stable static universe---but only the semblance of a steady state.

    In 1922 a Russian named Friedmann derives simplified equations (assuming a type of uniformity) from Einstein's complicated ones. The solutions of Friedmann's equations are either expanding or contracting.

    In 1929 Edwin Hubble announces that distant galaxies appear to recede---the farther the faster. Hubble didn't know about the changing scale factor in the metric of general relativity that makes space either expand or contract. It was a surprise confirmation of the model Friedmann had derived from GR.

    In the 1940s Gamow, Alpher, and Hermann took the expansion seriously enough to extrapolate it back to an earlier time of much higher density and temperature. In 1948 Alpher and Hermann predicted the remnant thermal glow would have been stretched out to microwave by the expansion of space and would still be detectable and would have a temperature of about 5 kelvin. (Eventually the temperature would turn out to be 2.73 kelvin but this was certainly in the ballpark.)

    In the 1960s Robert Dicke took that prediction seriously enough he decided to look for the microwave background. In 1965 a postdoc of Dicke named Peebles published an estimate of about 10 kelvin. Word spread and before Dicke and Peebles could get their antenna working, some other people Penzias and Wilson reported having already detected the microwave background as unexplained "noise"---at roughly the predicted temperature.

    Finding the CMB gave credibility to the expansion model of the universe partly because of serendipity. The accidental un-looked-for elements of the story helped convice people. Einstein hadn't wanted expansion but his equations predicted it (or contraction) almost to spite him. Hubble didn't know that about Einstein's model when he found the galaxies were receding. Gamow and others extrapolated back to a hot time and predicted remnant microwaves, which Penzias and Wilson then realized they had observed without knowing of the prediction!

    A more complete account would have to talk about how the abundance of different chemical elements fits into the story and about what has been learned by mapping slight temperature variations in the CMB---the COBE results of the 1990s and the WMAP results coming in at present. There is the further coincidence that the angular size of the temperature variations is consistent with spatial flatness---one could see it as a continuation of the run of luck this idea has been having.
     
  2. jcsd
  3. Jul 21, 2003 #2
    The Penzias and Wilson story.

    They were working on a new low noise amplifier for the, I beleive it was Cherryhill, research facility antenna. Naturally doing everything they could to eliminate noise. Most RF noise, like television snow, is actually produced in the amplifier itself. They pointed the antenna at the "darkest" parts of the radio sky but they still had this noise that couldn't be eliminated. Their tests and calculations said it wasn't coming from the thermal noise of the antenna, the atmosphere, or the amplifier. They finally decided it had to be coming from space itself and that it came from every direction.
     
  4. Jul 21, 2003 #3

    marcus

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    fixing big bang deficiencies

    Exerpt from Lineweaver:
    << Big Bang: Guilty of Not Having an Explanation

    “...the standard big bang theory says nothing about what banged, why it banged, or what happened before it banged. The inflationary universe is a theory of the “bang” of the big bang.” -– Alan Guth (1997).

    Although the standard big bang model can explain much about the evolution of the Universe, there are a few things it cannot explain:

    • The Universe is clumpy. Astronomers, stars, galaxies, clusters of galaxies and even larger structures are sprinkled about. The standard big bang model cannot explain where this hierarchy of clumps came from–-it cannot explain the origin of structure. We call this the structure problem.

    • In opposite sides of the sky, the most distant regions of the Universe are at almost the same temperature. But in the standard big bang model they have never been in causal contact-–they are outside each other’s causal horizons.
    Thus, the standard model cannot explain why such remote regions have the same temperature. We call this the horizon problem.

    • As far as we can tell, the geometry of the Universe is flat–-the interior angles of large triangles add up to 180 degrees. If the Universe had started out with a tiny deviation from flatness, the standard big bang model would have quickly generated a measurable degree of non-flatness. The standard big bang model cannot explain why the Universe started out so flat. We call this the flatness problem.

    • Distant galaxies are redshifted. The Universe is expanding. Why is it expanding? The standard big bang model cannot explain the expansion. We call this the expansion problem.

    Thus the big bang model is guilty of not having explanations for structure, homogeneous temperatures, flatness or expansion. It tries--but its explanations are really only wimpy excuses called initial conditions. These initial conditions are:

    • the Universe started out with small seeds of structure
    • the Universe started out with the same temperature everywhere
    • the Universe started out with a perfectly flat geometry
    • the Universe started out expanding

    Until inflation was invented in the early 1980’s, these initial conditions were tacked onto the front end of the big bang. With these initial conditions, the evolution of the Universe proceeds according to general relativity and can produce the Universe we see around us today. Is there anything wrong with invoking these initial conditions? How else should the Universe have started? The central question of cosmology is: How did the Universe get to be the way it is? Scientists have made a niche in the world by not answering this question with “That’s just the way it is.” And yet, that was the nature of the explanations offered by the big bang model without inflation.>>

    Lineweaver is a prominent representative of mainstream cosmology and is here presenting the consensus or "concordance" model of the cosmos---there has been a convergence of expert opinion and he speaks for it. Notice that in his view the big bang is not an explosion from a point---the U begins flat: the locus of the time-zero singularity is normally assumed to be an infinite flat 3D space at a particular instant of time.

    The space could have begun contracting at time zero, but the consensus view is that it began expanding instead. One of the problems Lineweaver cites is simply Why? Why did it begin expanding at time zero instead of contracting?

    LQG makes inflation scenarios (and some highly imaginative "colliding brane" alternatives to inflation) unnecessary. It takes care of the same problems about the big bang that the inflation scenario was invented to solve, without invoking inflation or wild speculative stuff. It does this by removing the classical singularity at time zero and letting an earlier contracting phase evolve smoothly into an expanding phase at time zero. The quantized model predicts the prior contracting phase and takes care of the transition from contraction to expansion, without prompting. Prior contraction provides the observed flatness and solves the temperature problem. So in a sense the problems which inflation was devised to solve were phony problems----they were artifacts of the classical Einstein equation which appear to vanish when that equation is quantized.
     
  5. Jul 22, 2003 #4

    Phobos

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    (my notes excerpted from various sources)

    1916 – Einstein developed foundations

    1922 – Alexander Friedmann's model of an expanding universe

    1923 – Hubble's work with Cepheid variable in M31 helped realization that “spiral nebulae” were actually other galaxies (universe was much bigger than previously believed)

    1929 – Hubble finds redshift & suggested expanding universe

    1964/5 - - Bell Telephone Laboratories scientists, Arno Penzias and Rovert Wilson, stumbled on discovery using their experimental radio antenna that microwaves are coming in from all directions (Cosmic Microwave Background). Big Bang moved from a hypothesis to a theory.

    1980 - - Alan Guth (MIT), Andre Linde, et al. propose Inflation Theory (refinement/addition to Big Bang Theory)

    1990/1992 - - COBE satellite confirms Cosmic Background Radiation & detects tiny temperature variations (possible seeds for early galaxy formation). Hawking called it the “discovery of the century, if not of all time”.

    1998 - - Balloon Observations of Millimetric Extragalactic Radiation and Geophysics data provide more detail for CMB temperature data. Data suggest that the universe is "flat".

    2001/2002 - - confirmation of polarization of CMB using the Degree Angular Scale Interferometer in the Antarctic

    2003 - - WMAP satellite image of CMB 35 times sharper than COBE
     
  6. Jul 22, 2003 #5

    Phobos

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    The theory is certainly due to be refined and expanded* upon. But I'm not betting any money on it being replaced completely.

    * excuse the pun :wink:
     
  7. Jul 22, 2003 #6

    marcus

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    Re: Re: brief perspective on notion of the big bang

    Talking about our subjective feelings----I have a lot of confidence in the basic expanding universe model: the Friedmann equations and the FRW metric. And the model seems elegant spare beautiful to me.

    The place I think change will come is that so far the model has a broken timeline----only half the timeline is there and this is an artifact of the classical Friedmann (originally Einstein) equations. This classical model only goes back to a certain time and then blows up. It doesnt "need" to and the quantized version currently being developed does not blow up. So there is no need to think of everything beginning at some initial moment or to picture mechanisms to generate "initial" conditions in a split second.

    Please think about this and give me your response: what do you think happens to the "big bang" concept when the timeline is extended back thru the classical singularity.

    The quantum version of the model predicts a prior collapsing phase which one of the Penn State people working on this has described as a "bounce"---indeed the computer-generated graphics of evolution thru time zero do make it look rather like a bounce but the mechanism is a difference-equation model.
     
  8. Jul 22, 2003 #7
    Marcus, I find that prior contracting phase very suspicious. It suggest an anterior universe, but in the starting of our universe there were only light elements such hydrogen or helium. Why not heavier elements of an anterior universe?

    I will post here my notes about Big Bang (if someone finds some error, say it)

    -10^-43 s after time zero: Plank era. Temperature of the universe of 10^32 K. Gravity separates of the others forces

    -10^-35 s a.t.z.: Strong force separate of the others forces. Temperature of the universe: 10^27 K

    -10^-12 s a.t.z.: Weak force separates of electromagnetism

    -0.01 s a.t.z. : Same number of protons and neutrons in the universe.
    Temperature of the universe is 10^11 K

    -1 s a.t.z: Neutrino decoupling

    -1.1 s a.t.z.: The proportion neutrons/protons is: 24 neutrons for every 76 protons

    -100 s a.t.z: Protons and neutrons form the first nuclei

    -500000 years a.t.z: Formation of the first atoms
     
  9. Jul 22, 2003 #8

    marcus

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    Meteor, I do not think what you say about light elements is a problem since at the time of transition things are supposed to have been so hot that atomic nuclei would not have been able to exist.

    In the Loop Quantum Cosmology calculations I have seen (Ashtekar summarizes the main results arxiv.org/math-ph/0202008 but it is the same in a number of papers by others as well) the density and one assumes the temperature are bounded by extremely high values.
    they are bounded, which means the physical model does not break down, but they achieve Planckian values which is to say very extreme. Planck temperature is for example 1032 kelvin. It is hard to imagine anything like an atomic nucleus existing without being resolved into constituent quarks or pure radiation.

    Thanks for posting the timeline! Perhaps it could be used to construct a scenario for the antecedent contracting stage?

     
  10. Jul 23, 2003 #9

    Phobos

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    to add to meteor's timeline...

    1E-36 seconds - - Start of Inflationary Epoch. First quarks.

    1E-34 seconds - - Inflationary period slowed down. Universe contained only elementary particles like quarks and electrons.

    1E-6 seconds - - Quarks combined to form the first protons and neutrons.

    1 millisecond - - Universe is still about 30 million times hotter than the surface of the sun and 50 billion times denser than lead.

    3 minutes - - First atomic nuclei.

    300,000 years - - First atoms. Universe cool enough and thin enough to become transparent to light.
     
  11. Jul 23, 2003 #10

    jcsd

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    Yet another timeline (overlapping a bit with the other timeline) for your eyes (the figure in the brackets is the redshift):

    0 Singularity, big bang (infinite)

    1E-43s Planck era, particle creatio (1E32)

    1E-36s Inflation, generation of density flucations (1E28)

    1E-10s Hadronic era, production of quarks (1E12)

    1s Leptonic era, anihilation of electron-positron pairs (1E10)

    60S Radiation era, nucleosyntheis of helium and deuterium (1E9)

    1 week Radiation thermalizes befpre this epoch (1E7)

    10,000 yrs Matter era, universe becomes matter dominated (1E4)

    300,000 yrs decoupling era, Universe becpmes transparent (this is when CMBR is emitted) (1E3)

    1-2bn yrs galaxy formation begins (10-30)

    2-3bn yrs galaxy clustering begins (5)

    3.1bn yrs first star forms (4)
     
  12. Jul 23, 2003 #11

    marcus

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    For the most part I have made use of jcsd's numbers, with some changes suggested by Meteor. Thank you all!
    The history of expansion is one of decreasing temperature: space expands causing temperature to decrease and this, in turn, allows increasingly familiar conditions and forms of matter to emerge---a kind of "condensation" of normality as space expands.

    The history of contracting space is essentially the reverse----as the energy density and temperature of the universe rise the normal conditions and materials one by one "evaporate". So I have used the temperature marks which jcsd gave, but in reverse order.
     
    Last edited: Jul 23, 2003
  13. Jul 23, 2003 #12
    Only want to say that was always believed that star formation started passed 1 billion years, but WMAP changed all and know is believed that star formation started 200 million years after Big Bang
     
  14. Jul 23, 2003 #13

    jcsd

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    Marcus the time figures I gave should be in the correct order, as should the redshift figures.

    The figure in the brackets isn't the temperature (though obviously being a function of the size of the observable universe it's related) it's how much an em wave emited in that era would be redshifted if it arrived on Earth today.
     
  15. Jul 23, 2003 #14

    marcus

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    How shall I change the timeline? What is the redshift for 200 million years? What shall I do about the dates for galaxy formation and galacit cluster formation? I have to go out now
    and can't do the research for this, Meteor, so please give me
    some suggestions! Will be back inside of one hour.
     
  16. Jul 23, 2003 #15

    jcsd

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    Those figures are about ayear or two old.

    re star formation: it's important to note that the figure of 3.1 bn yrs is not when stars started to form, but when they were formed.
     
  17. Jul 23, 2003 #16

    marcus

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    I made some corrections, particularly near the end of the list of "after time zero" dates, are they in proper order now?

    I have used a standard equation in cosmology that
    the ratio of scale factors a(temit)/a(treceive) is equal to 1/(z+1)
    to use your redshift figures to calculate the scale factor in each case

    the time the light is received is always the present so that a(treceive) = 1

    so the scale factor in each case is simply given by 1/(z+1)
     
    Last edited: Jul 23, 2003
  18. Jul 23, 2003 #17

    jcsd

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    Sorry, by the right order I meant that my figures should be correct to within a factor of 10.
     
  19. Jul 23, 2003 #18

    marcus

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    Now I see, order means order of magnitude! Thanks for
    clearing that up, and also for the timeline. I am now wondering
    how to make the correction suggested by Meteor, to allow for
    star formation earlier than was believed prior to WMAP.

    There is a major paper of Feb 2003 results from WMAP at this address:
    http://arxiv.org/astro-ph/0302207
    so I will look there
     
  20. Jul 23, 2003 #19
    Last edited: Jul 23, 2003
  21. Jul 23, 2003 #20

    marcus

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    I am thankful to you both for your help. Meteor, I will take your advice here.
    It seems to me that 600 million years should correspond to
    a redshift of about 7
    and therefore the scale factor a(t) would be 1/8

    I will edit the above to make some corresponding change.
    and I'll try not to overstate the precision or be too definite.
    It is surely understood that it is only a rough schematic.

    In calculating the redshift I have used "Ned Wright's Cosmic Calculator"
    http://www.astro.ucla.edu/~wright/CosmoCalc.html
    which was last updated in March 2003 and uses the latest
    parameter values. He is a member of the WMAP team
    so I tend to trust what the calculator says.
     
    Last edited: Jul 23, 2003
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