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Thread: Two unrelated questions on the Big Bang and our cosmologic horizon

  1. #1 Two unrelated questions on the Big Bang and our cosmologic horizon 
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    Hi there, I'm new to this website and this is my first post. Regarding the Big Bang, my understanding used to be that the Big Bang contained all the energy of the universe, like matter and galaxies etc, which expanded outward in all directions and as it expanded, the energy cooled and eventually formed the first atoms. However, recently in the past few years I've learned that the matter in our universe is actually the "leftovers" of the Big Bang. When the energy cooled enough to form atoms, it actually produced almost equal amounts of matter and antimatter which annihilated into pure radiation. There was a tiny tiny fraction of a percent more of matter than there was antimatter, which did not annihilate and which now makes up all the matter in the universe as we know it. The question I have is, is it actually incorrect to say that the Big Bang contained all the energy of all the matter in the universe? Would the correct description be, the Big Bang contained billions of times more energy than all the energy contained in all the matter in the universe as we know it? It is kind boggling enough to think that all the energy of our universe was compressed into a region smaller than a single atom, but to think that it actually contained BILLIONS of times more energy..... Is my thinking/understanding correct or is there a flaw in my reasoning? My next question has to do with our cosmologic horizon. I understand that the further we look in space, we are looking back in time since it has taken light a long time to travel the vast distances of space to reach us here on earth. I also understand that space in between galaxies and galaxy clusters has been expanding as well. Therefore light that may have left a galaxy 10 billion years ago may have taken 13 billion years to reach earth, which would imply that that particular galaxy is now more than 13 billion light years away. What I'd like to know is, how far away was the cosmic horizon when it "originated," 300,000 years after the Big Bang when matter cooled enough to clear the electron "fog" and release the CMB? Does this imply that, as far as we possibly can see in the universe, there is matter everywhere. Since we can't see beyond the horizon, we can never know the true extent of the cosmos. It could be many magnitudes larger than what we can observe, similar to the Earth's horizon. Is that correct or is my understanding of the cosmic horizon wrong? And further, if we don't know the extent how do physicists calculate the percentages of matter/energy/dark matter/dark energy and how can they tell that we are very close to the critical values of omega=1?I have an engineering background so I can generally understand more technical explanations. Would anyone be able to recommend some good books/resources on the Big Bang, cosmology, etc? This stuff absolutely fascinates me.


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    I am going to answer most of your second question, and leave your first and last for others to address.

    Quote Originally Posted by Dr. Blunt View Post
    My next question has to do with our cosmologic horizon. I understand that the further we look in space, we are looking back in time since it has taken light a long time to travel the vast distances of space to reach us here on earth. I also understand that space in between galaxies and galaxy clusters has been expanding as well.
    This is a good start.

    Quote Originally Posted by Dr. Blunt View Post
    Therefore light that may have left a galaxy 10 billion years ago may have taken 13 billion years to reach earth, which would imply that that particular galaxy is now more than 13 billion light years away.
    I know what you might have meant to say here, but the way you said it is confusing. If the light left the galaxy 10 billion years ago, then it would have taken 10 billion years to reach Earth! It would, however, have travelled more than 10 billion light-years (which is a measure of distance, rather than time) to reach Earth, due to the expansion of the universe.

    Let me put some actual figures to your example, based on the current cosmological model.

    For a galaxy whose light has been travelling for 10 billion years:
    We are seeing that galaxy as it was when the universe was only 3.8 billion years old.
    That galaxy was around 5.7 billion light-years away when that light was emitted.
    Today, due to the expansion of the universe, that galaxy will be around 16.2 billion light-years away.

    Your galaxy has a redshift of z=1.8, and redshift represents the scale factor of the universe in the form 1+z, so the universe is now 2.8 times larger than it was when that light was emitted.

    Quote Originally Posted by Dr. Blunt View Post
    What I'd like to know is, how far away was the cosmic horizon when it "originated," 300,000 years after the Big Bang when matter cooled enough to clear the electron "fog" and release the CMB?
    Around 42 million light-years. Yes, that's million, not billion.

    The CMB has a redshift of z=1089, which means the universe is now 1090 times larger than it was when the CMB was released. 42 million light-years x 1090 = 46 billion light-years, the estimated current radius of the observable universe.

    Quote Originally Posted by Dr. Blunt View Post
    Does this imply that, as far as we possibly can see in the universe, there is matter everywhere.
    Yes, it does.

    Quote Originally Posted by Dr. Blunt View Post
    Since we can't see beyond the horizon, we can never know the true extent of the cosmos. It could be many magnitudes larger than what we can observe, similar to the Earth's horizon. Is that correct or is my understanding of the cosmic horizon wrong?
    Your understanding is correct.

    When the CMB was released, it was released everywhere in the universe, as atoms formed everywhere. The CMB has been passing this place ever since, and the CMB that reaches us today was originally released only 42 million light-years away. In the past we have been hit by CMB that was released less than 42 million light-years away, and in the future we assume we will continue to be hit by CMB that was released more than 42 million light-years away.


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    Quote Originally Posted by Dr. Blunt View Post
    Hi there, I'm new to this website and this is my first post. Regarding the Big Bang, my understanding used to be that the Big Bang contained all the energy of the universe, like matter and galaxies etc, which expanded outward in all directions and as it expanded, the energy cooled and eventually formed the first atoms.
    It's probably wrong to speak of "the Big Bang" as if it's a thing. If one extrapolates backwards in most models, one reaches a singularity, a place in the physics where we cannot go further back and in which the mean distance between any two points is zero. This is not good physics, at least not yet. We have a theory of the universe from a hot, dense state that evolved into the universe as we see it now.
    However, recently in the past few years I've learned that the matter in our universe is actually the "leftovers" of the Big Bang. When the energy cooled enough to form atoms, it actually produced almost equal amounts of matter and antimatter which annihilated into pure radiation.There was a tiny tiny fraction of a percent more of matter than there was antimatter, which did not annihilate and which now makes up all the matter in the universe as we know it.
    Well, maybe. Clearly there is more matter than anti-matter. The exact history of this is not clear.
    The question I have is, is it actually incorrect to say that the Big Bang contained all the energy of all the matter in the universe? Would the correct description be, the Big Bang contained billions of times more energy than all the energy contained in all the matter in the universe as we know it?
    Energy does not seem to escape the universe or be destroyed. If there was a great matter/anti-matter interaction in the very early universe, then that energy is still here.
    My next question...
    See SpeedFreek's anwer.
    And further, if we don't know the extent how do physicists calculate the percentages of matter/energy/dark matter/dark energy and how can they tell that we are very close to the critical values of omega=1?
    First, there is no border to the universe, there is only the maximum length of a path through the universe. Nothing can take a path that would lead it outside the universe and no path can enter the universe from the outside. So any horizon is only a limit for a particular point given the spacetime structure of the universe around it.

    Second, astronomers use a number of techniques to figure out the structure and composition of the universe. Some of these measure the distance to the origin of the CMB.
    I have an engineering background so I can generally understand more technical explanations.
    Engineering backgrounds are generally red flags when it comes to relativistic physics. It might be that engineers are taught a lot of shortcuts, general cases, and methodologies that are fine for a lot of applied physics for applications on the scale of human activity and architecture but that are not great for theoretical science. I don't mean to make offense, but there seems to be such a disproportionate amount of engineers denying contemporary physics that I would warn you to be extra careful.
    Would anyone be able to recommend some good books/resources on the Big Bang, cosmology, etc? This stuff absolutely fascinates me.
    A good reference, Ned Wright's Cosmology Tutorial: Ned Wright's Cosmology Tutorial

    I'm not a good one for general intro books. I think Kirshner's book, The Extravagant Universe did a good job. I'm not a big fan of Brian Greene, too speculative.
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