Wikipedia:Reference desk/Archives/Language/2014 October 31

= October 31 =

Sentence understanding problem
Hello.

Please read the following two sentences.

According to the team, the universe is 13.798 ± 0.037 billion years old, and contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy. Also, the Hubble constant was measured to be 67.80 ± 0.77 (km/s)/Mpc.

Is the Hubble constant saying that the universe is 67.80 ± 0.77 (km/s)/Mpc old? If so, "(km/s)/Mpc" doesn't make sense.

I don't understand?

(Russell.mo (talk) 18:14, 31 October 2014 (UTC))

What do you the highlighted bits mean?

This scenario is generally considered to be the most likely,[citation needed] as it occurs if the universe continues expanding as it has been. Over a time scale on the order of 1014years or less, existing stars burn out, stars cease to be created, and the universe goes dark.[34],§IID. Over a much longer time scale in the eras following this, the galaxy evaporates as the stellar remnants comprising it escape into space, and black holes evaporate via Hawking radiation.[34], §III, §IVG. In some grand unified theories, proton decay after at least 1034 years will convert the remaining interstellar gas and stellar remnants into leptons (such as positrons and electrons) and photons. Some positrons and electrons will then recombine into photons.[34], §IV, §VF. In this case, the universe has reached a high-entropy state consisting of a bath of particles and low-energy radiation. It is not known however whether it eventually achieves thermodynamic equilibrium.[34], §VIB, VID.

(Russell.mo (talk) 18:18, 31 October 2014 (UTC))
 * With regard to your second question, the symbol § stands for "section", so those notes are identifying the sections of the article cited in footnote 34 that contain the information. It appears to me that the first parts are roman numerals, so §IID would be Section 2D, §IVG would be Section 4G, and so forth. (For the benefit of other potential respondents, the article from which Russell.mo's passages are taken is Chronology of the universe.) Deor (talk) 19:35, 31 October 2014 (UTC)


 * With regard to the first question: No, Hubble's constant is not a measure of the age of the universe; it's a constant used in estimating the rate at which objects are receding from one another as a result of the expansion of the universe. "(km/s)/Mpc" means "kilometers per second per megaparsec". The current estimate of the age of the universe is given in the first sentence you quoted: 13.798 ± 0.037 billion years. (Any physicists here are welcome to correct anything I may have bollixed up in this explanation. I'm definitely not a physicist.) Deor (talk) 20:20, 31 October 2014 (UTC)


 * Same here! -- (Russell.mo (talk) 04:28, 1 November 2014 (UTC))


 * On the first question, the Hubble constant is a measure of how fast the universe is expanding; the units are "Rate of expansion per distance from the observer". That is, for a given distance from you (the observer) the objects that distance away seem to be all receding at a constant rate, and the rate of expansion increases steadily with increasing distance.  The unit itself measures the rate of expansion in kilometers per second (or km/s), and the distance from the observer is measured in Megaparsecs, or Mpc (where a Megaparsec is a million parsecs), so the unit for the constant is "Kilometers per second per megaparsec" or "(km/s)/Mpc".  What the constant is saying is that for an object 1 megaparsec away, it appears to be receding from you at a speed of 67.80 km/s.  For objects 2 megaparsecs away, they would recede at twice that speed, and so on.  -- Jayron  32  20:28, 31 October 2014 (UTC)


 * That's not quite correct. It's only an average. Objects have different velocities relative to us, some approaching, most receding. The Andromeda Galaxy, for example, is going to get very, very up close and personal in about 4 billion years. Clarityfiend (talk) 02:25, 1 November 2014 (UTC)


 * Lol, We'll be dead by that time. I think I get the idea, its the average measurement from hubble spacecraft (if an observer is in it, or the hubble spacecraft is an observer itself) how fast the universe is expanding... Am I Right? (Russell.mo (talk) 04:28, 1 November 2014 (UTC))


 * Not quite. Edwin Hubble came up with his constant decades before there were any spacecraft (at least human spacecraft - mwahahahaha). He noticed that objects, on average, were moving away from us faster the further away they were. The constant just tells us how much that average speed changes per megaparsec of separation from us. (It's not just us; if you were in a galaxy far, far away, it would be the same.) There is no Hubble spacecraft, AFAIK, just the Hubble Space Telescope, which is named after him. Clarityfiend (talk) 05:19, 1 November 2014 (UTC)


 * Well, I guess, I understand. -- (Russell.mo (talk) 17:40, 1 November 2014 (UTC))
 * Is that another way of saying "I don't understand"? --  Jack of Oz   [pleasantries]  19:55, 1 November 2014 (UTC)
 * Lol -- (Russell.mo (talk) 07:08, 2 November 2014 (UTC))


 * To be entirely accurate, it wasn't Hubble that came up with it in isolation (as though the thought sprang fully formed from his mind with no antecedents). The notion of an expanding universe was implicit in the Einstein field equations of General relativity, though Einstein was uncomfortable with the notion enough to introduce his Cosmological constant to counteract the expanding universe his equations told him should be happening.  So Einstein himself deserves some credit, though accidentally.  Secondly, a whole slew of scientists come between Einstein and Hubble and the proof of the expanding Universe, probably most importantly Georges Lemaître, but also not to be ignored include Alexander Friedmann, Howard P. Robertson, Arthur Geoffrey Walker.  Hubble's contribution wasn't to determine the universe was expanding, nor was it to provide the law and constant that bear his name; his main contribution was the connection between Redshift of distant objects and Lemaître prediction of the expanding universe.  Basically, Hubble found the evidence for the predictions, (and he also had better PR) so he's generally better remembered.  But science doesn't happen in a vacuum; and these scientists and mathematicians worked among a community of like individuals, many of whom made important, incremental contributions to our knowledge in this area.  -- Jayron  32  20:22, 1 November 2014 (UTC)


 * I understand how they came up with the 13.8 bya measurement. I don’t understand how Hubble came up with his constant. If he wasn’t in a spacecraft, then he was on Earth. If he measured it from Earth then he is saying the constant can be understood from any part of the galaxy. The rate of expansion, I guess measured from the ‘primodial atom’ point to the farthest expansion point.
 * The blast, big bang, the so called cosmic inflation that was left after the blast, did it take 377,000 years for the blast to come to the state which left the trace of the cosmic inflation? What I mean to say, did one blast of an atom spread throughout space for 377,000 years?
 * And what the difference between 'space' and 'universe'. I thought universe is what everything is, from ‘primodial atom’ to the farthest expansion point. But in 'space' article says that the space is created during the 'big bang'. So what was it before, “white body”? White/blue/green/yellow/maroon back ground rather than black background? I know no one knows, I'm just trying to understand the difference between the word 'space' and 'universe'. Is space is where universe is created? -- (Russell.mo (talk) 07:08, 2 November 2014 (UTC))
 * No, the universe is mainly made up of space, with just the occasional bits of matter dotted around in space. Both space and time were created in the big bang, most likely from a singularity (a mathematical concept which is almost meaningless to a layman in this context, but basically just a single point).  The rate of expansion is the same measured from any point in the universe because each current point was the centre 13.8 bya.  The universe is not expanding into anything, it is just expanding, with nearly every galaxy moving away from nearly every other galaxy because of the metric expansion of space.  The 377 000 years is just the time it took for the universe to cool sufficiently for atoms to form.  Before that it was just plasma.    D b f i r s   08:29, 2 November 2014 (UTC)


 * If the universe is not expanding into any preexisting space, that is if space itself is expanding, is in fact created by the expansion of the universe, then how come this has any large scale effect in the end? Since space is expanding is it not true that not only the distances between galaxies should be increasing but also the distance between all of the elementary particles in the universe away from each other? In other words why would galaxies only be pulling away from each other? The particles those galaxies are composed of should also be pulling away from each other, thus canceling any effect. The way this concept is commonly explained does not make much sense. Contact Basemetal   here  09:05, 2 November 2014 (UTC)


 * That's a good question, and one that I used to wonder about, but apparently gravity within a galaxy, and atomic forces within matter, prevent the expansion in some way, so it is only the space between galaxies that expands. I'm not entirely happy with the explanation myself, and I suspect that there is something else going on that we don't yet fully understand.    D b f i r s   10:43, 2 November 2014 (UTC)


 * Yes there is a paradox in this explanation. If it's the distance that allows the galaxies to move away from each other then what happened when the universe was much smaller? How did the expansion even get started? And why are there galaxies in the first place? Why isn't matter uniformly distributed in the universe? Contact Basemetal   here  15:43, 2 November 2014 (UTC)


 * The expansion seems to be governed by dark energy, and it seems that the expansion is accelerating, and the amount of this mysterious dark energy is increasing at a corresponding rate while the amount of matter remains constant. Some other possibilities are suggested in Dark energy.  The non-uniformity is usually explained by resonances in the early plasma (imagine the universe ringing like a bell) so that when atoms formed, some were closer together than others. See Structure formation for a technical explanation.    D b f i r s   08:42, 3 November 2014 (UTC)


 * Distances are measured indirectly by measuring the apparent brightness of a "standard candle", an object of known intrinsic brightness. Speeds are measured by redshift. —Tamfang (talk) 08:21, 2 November 2014 (UTC)


 * Kilometers and megaparsecs are both units of length, so km/Mpc is a dimensionless ratio; thus Hubble's constant is in units of inverse time – so its reciprocal gives the time since the expansion began, assuming that its speed has been constant. —Tamfang (talk) 08:21, 2 November 2014 (UTC)