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Is this correct?

  1. Sep 19, 2009 #1
    I am reading this book by a cosmologist where he is talking about how the universe must be pretty old to be able to produce observers to figure out how old the universe is. This makes sense. The universe must be at least billions of light years in size to have had the time to produce the elements to produce life. The point I am wondering about is when he says that a universe that was only the size of the Milky Way would only be a month old. The actual universe is 14 billion years old. First of all, is the month claim accurate? Does this mean that we know that something the size of the Milky Way can form in about a month? The Milky Way took a month to make?(the space of it) Where does inflation come into the equation?

  2. jcsd
  3. Sep 19, 2009 #2


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    Inflation is the short answer. The very early universe expanded at superluminal velocity.
  4. Sep 19, 2009 #3
    Ok, but what percentage of the milky way's size is because of faster than light inflation? And how much of its size is from different forms of expansion?
  5. Sep 20, 2009 #4


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    I really dislike this terminology. Expansion isn't a velocity. It's a rate (or velocity per distance, whichever you prefer). Any amount of expansion is "superluminal" at large enough distances.

    What separated out inflation was that it was a period of exceptionally fast exponential expansion.

    Anyway, the claim being made in that book is a somewhat different claim, one that inflation does not answer. The idea is simply this: if our universe is a thermal fluctuation, it is a pretty darned big thermal fluctuation (the entropy of the early universe was absurdly small compared to the current entropy, or the eventual entropy of our region of space-time). One early concept was that this was just a natural thermal fluctuation: if you wait long enough, thermal fluctuations of all sizes will occur. So eventually you will get a region with tremendously minuscule entropy, like our own had.

    However, it's much easier to make a small fluctuation than a large one. Small fluctuations happen tremendously more frequently than large ones. Instead of making a whole universe, it's a much tinier fluctuation to just make a single galaxy instead. Heck, it's an even smaller fluctuation to make a single brain. A brain that has memories of a long existence, and even has some sensory inputs that make it think there is something around it. This brain immediately dissolves back into the thermal fluctuation from whence it came, but it thinks it exists, and observes a reality.

    Fortunately, we can be pretty sure that we are not such brains: nearly all of these "Boltzmann Brains" would observe a horribly disordered, chaotic universe. In order to observe an orderly universe, in order for our past experience to have a chance of predicting future outcomes, it only really makes sense that we are actually real, that our early universe actually had extremely low entropy.

    But then this brings us right back to the beginning: why did our early universe have such low entropy? The discussion above proves that it wasn't simply a rare thermal fluctuation. But then what was it? Well, there are a number of ideas. Sean Carroll somewhat recently wrote a popular science article on the subject:
  6. Sep 20, 2009 #5
    I suspect that the book is referring not to something physical as per the milky way but 'space'. Matter cannot travel faster than the speed of light but 'space' can.
    ergo, due to inflation, 'space' could well have been the size of the milky way - but the milky way wasn't there - yet! The Milky Way took a long time to condense out of this 'space'.
    Some books state that the after a tiny fraction of a second the universe would fit into a suitcase. This doesn’t mean that suitcases were discovered and made a tiny fraction of a second after the big bang! Its just a way of giving some perspective to the rate of inflation.
  7. Sep 20, 2009 #6
    Surely it is more or just as likely that these "Boltzmann Brains" would have false memories of a low entropy universe rather than observing the disordered universe they actually exist in. Especially as presumably a fluctuation which allows for some sort of sensory input of the 'actual' universe would have to be larger.
  8. Sep 20, 2009 #7


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    Nope. There are so vastly many more ways for such brains to "observe" a disordered universe that this is the more likely probability.
  9. Sep 21, 2009 #8


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    Hmmm, you may have the more correct interpretation of this section of the book.
  10. Sep 21, 2009 #9
    Welcome to Physics Forums. :smile: May I please have the author's name and the title of the book.

    Thank you,
  11. Sep 21, 2009 #10
    John D Barrow. The Constants of Nature.

    Here is the quote:

    "We have seen that the process of stellar alchemy takes time-billions of years of it. And because our universe is expanding it needs to be billions of light years in size if it is to have enough time to produce the building blocks for living complexity. A universe that was only as big as our Milky Way galaxy, with its 100 billion stars, would be little more than a month old."
  12. Sep 21, 2009 #11


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    Okay, maybe both of us got it entirely wrong. This doesn't make any sense to me at all.
  13. Sep 21, 2009 #12
    Just looks like the authors trying to say that the universe is very big opposed to very small because we have 2 observables... expansion rate and the rate at which stellar alchemy occurs. So because there are materials available to create what exist (observable to us) we can conclude that stellar alchemy has been occuring for billions of years. Put that in with our expansion rate and boom you have a large universe as opposed to a milky-way sized universe.

    I don't know what the implications of this are in the book, if its of any use at all even (doesn't really seem that way...) nor have I read the book.

    I of course can completely wrong on this.
  14. Sep 21, 2009 #13
    Freeman, thank you for the information. John D. Barrow is a big fan on "Uncommon Descent" which serves the Intelligent Design Community. Barrow is into TELEOLOGY AND THE ANTHROPIC PRINCIPLES. Freeman, I'll be honest with you, I'm not an advocate for the Intelligent Design Movement. Naturally, I wouldn't be a fan of John D. Barrow.

    Thanks for responding to me. :smile:
  15. Sep 21, 2009 #14
    What do you mean he is a big fan? He is not an advocate of intelligent design. And plenty of top scientists use the Anthropic Principle. From Weinberg to Hawking to Susskind.
  16. Sep 21, 2009 #15
    They (proponents of Intelligent Design) consider him to be their fan (follower) because of some things he has written, according to what I have read on the website Uncommon Desent.
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