At the beginning of a video on youtube titled "history of the world in 2 hours", it is said that "every thing begins with this tiny bundle of energy smaller than an atom". I want to know what it means. atom is matter. energy is different from matter and is measured in joules where as size is measured in volume units. How can one say energy smaller than an atom?
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While this isn't your question, it's related to the "started at a point" that many shows imply, even though it's perhaps not accurate. http://physics.stackexchange.com/questions/136860/did-the-big-bang-happen-at-a-point – userLTK Nov 27 '16 at 09:04
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1I'm voting to close this question as off-topic because it does not appear to be about astronomy – SE - stop firing the good guys Nov 27 '16 at 09:43
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1I am curious to know how this question is outside the "early-universe" tag? – Seetha Rama Raju Sanapala Nov 27 '16 at 09:55
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Your question seems to be about definitions more than astronomy. Defining the state of the early universe as "energy" isn't a statement of joules so much as a statement of hot primordial energy-matter soup, so to speak, but so hot, that there is no matter, so it's just called energy as a statement of convenience. If you put enough energy into matter, the matter-energy distinction becomes far less clear. They're not just related but they become fully interchangeable. Calling the Early universe energy is just one way to phrase it. Density is another method, as is temperature. – userLTK Nov 27 '16 at 16:40
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1This is plainly a question about the Big Bang though it's very poorly phrased. But it is an astronomy question as it's about cosmology. – adrianmcmenamin Nov 27 '16 at 20:09
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I would agree with @adrianmcmenamin. While the question could do with some sprucing up to ask a clearer, more focused question, it is clearly a question about specific components of cosmological theory, specifically what does it mean to have all the energy of the universe confined to a point and how does one conceptualize such a scenario. It does not deserve to be down-voted or closed. – zephyr Nov 28 '16 at 01:52
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It's energy density, which easily maps to physical volume. – Carl Witthoft Nov 28 '16 at 14:49
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What the video is clumsily alluding to is that the universe was very much smaller than it is today. However, if the universe is infinite, then it was still infinite at all times in the past. All one can say is that the density was much higher.
Sometimes commentators are more precise. One can define a size scale for the currently observable universe and ask how that volume of space evolved with time. This of course assumes an accurate knowledge of cosmology and the relevant cosmological parameters.
A ball park number (see here) is that the universe has got bigger by a factor of $10^{30}$ since the end of the inflationary epoch (at $t \sim 10^{-32}$ s; which is not fully understood yet). Thus the current observable universe, of size $10^{27}$ m, was contained within a diameter of 1 mm at the end of the inflationary epoch. Prior to the inflationary epoch it must have been compacted into a much smaller volume than this - and I think this is the origin of the statement in your question.
The reason that your source talks about energy is because that is in what form the density of the universe was at that epoch. It was only later that this energy started to "freeze out" into various forms of matter. It is now commonly supposed that the Higgs mechanism or some form of it, is what gives elementary particles the property of a rest mass that might serve as a definition of "matter". The Higgs mechanism kicks in at the epoch of electroweak symmetry breaking (at temperatures of about $10^{15}$ K), giving paticles mass. Prior to this the density of the universe was in the form of massless entities that you might refer to as "energy". This symmetry breaking occurs after the end of the inflationary epoch at $t \sim 10^{-12}$s, and therefore when the currently observable universe was larger than 1 mm. So prior to this, and certainly when the currently observable universe was "smaller than an atom", the contents of the universe could be described in term of energy (without rest mass).
