A space expansion question

[SpeedFreek]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'><font color="#0000ff">I have seen some thought to the effect that the universe might have a radius of 46 billion light years, but that is NOT the observable universe.The universe is thought to be about 13 billion years old (i.e. since the Big Bang) that would place the radius of the observable universe at no more than 13 billion light years. </font><br /> Posted by DrRocket</DIV></p><p>It all depends on what distance measure you use, but all these figures <strong>DO</strong> refer to the observable universe, i.e. the sphere of space around us all the way out to the "surface of last scattering", when the CMBR was emitted. I suppose it depends on your definition of observable universe.</p><p>The figure of 13.7 billion light years is actually the age of the universe in terms of <strong>light-travel time</strong>. This is a common measure used in a universe where we are effectively looking back in time as we look outwards, but it is not a physical measurement of the radius of the universe in terms of distance. We cannot see objects anywhere near 13 billion light years away, in fact the most distant object we have imaged, a galaxy at a redshift of around z=7, was only 5.7 billion light years away when it emitted the light are now seeing. That is the distance as estimated using the angular diameter of that galaxy, i.e. how big it looks in the sky.</p><p>The <strong>angular diameter distance</strong> is the measure of where an object was when it emitted the light we are now seeing. As I said above, the most distant object we have imaged was under 6 billion ly away when it emitted that light. It emitted that light around 12.8 billion years ago. That galaxy is now estimated to be around 29 billion light years away. This measure is known as the <strong>co-moving distance</strong> and it represents how distant that galaxy is today, 12.8 billion years later.</p><p>The observable universe is estimated to have been around 40 million light years in radius when the CMBR was emitted, and to be around 46 billion light years in radius today. This means that we have received photons that were emitted with the CMBR at an original distance of 40 million light years away and have taken 13.7 billion years to reach us. The co-moving coordinate that was 40 million light years away when they were emitted is now 46 billion light years away. In the future we expect to receive CMBR photons that were more distant than 40 million ly when the CMBR was emitted, from co-moving coordinates that will be more than 46 billion light years away when we receive them. </p><p>The most distant point from which we can have received CMBR photons represents the edge of our observable universe and it is known as the surface of last scattering, or the particle horizon. That is how our observable universe is defined, and the 13.7 billion light years is only a measure of the time that light has had to travel in. Some time in the past our observable universe <em>was</em> actually that radius, but that is arbitrary in this context.&nbsp;</p><p>These are the distance scales of the universe. <em>(The link is a little old and uses slightly high numbers, but it is close enough!)</em> </p> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

 

[derekmcd]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>It all depends on what distance measure you use, but all these figures do refer to the observable universe, i.e. the sphere of space around us all the way out to the "surface of last scattering", when the CMBR was emitted. I suppose it depends on your definition of observable universe.<br /> Posted by SpeedFreek</DIV></p><p>I really wish they could just come up with standard definitions and stick with them:</p><p>The Universe<br />observable universe<br />visible universe<br />cosmic horizon<br />universal event horizon<br />cosmic light horizon<br />particle horizon<br />cosmological event horizon<br />visible light horizon<br />light cone<br />hubble sphere<br />hubble volume<br />hubble radius<br />hubble limit<br />hubble constant<br />hubble flow<br />hubble expansion<br />hubble law<br />hubble parameter<br />comoving frame<br />comoving time<br />cosmological time<br />comoving coordinates<br />comoving distance<br />proper distance<br />cosmological proper distance<br />angular diamter distance<br />angular size distance<br />proper motion distance<br />luminosity distance<br />reshift<br />light travel time<br />look back time<br />light year<br />light year distance<br />parsec</p><p>Etc...&nbsp;</p><p>Sometimes when I'm reading or following a discussion, I just pay attention to the context in which the terms (rightly or wrongly) are being used.</p><p>&nbsp;</p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>

 

[BetweenMyths]

Could our universe be continuing to expanding because our universe and an anti-universe are attracting each other and exchanging matter-energy?&nbsp; i.e. matter/energy being exchanged with ant-matter/anti-energy. When the universes&nbsp;eventually touch - It's big crunch time.

 

[derekmcd]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Could our universe be continuing to expanding because our universe and an anti-universe are attracting each other and exchanging matter-energy?&nbsp; i.e. matter/energy being exchanged with ant-matter/anti-energy. When the universes&nbsp;eventually touch - It's big crunch time. <br /> Posted by BetweenMyths</DIV></p><p>What is an "anti-universe" and how would it exchange energy?&nbsp; </p><p>Matter/antimatter (no such beast as anti-energy) don't exchange energy... their interactions release energy.&nbsp;</p><p>A "big crunch" would only happen if there is enough mass within the universe that is greater than the critical density.&nbsp;</p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>

 

[DrRocket]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Could our universe be continuing to expanding because our universe and an anti-universe are attracting each other and exchanging matter-energy?&nbsp; i.e. matter/energy being exchanged with ant-matter/anti-energy. When the universes&nbsp;eventually touch - It's big crunch time. <br />Posted by BetweenMyths</DIV></p><p>You can hypothesize almost anything.&nbsp; But generally the universe is by definition everything with which whatever we are in can interact.&nbsp; So interaction with something outside the universe is not possible, essentially by definition.&nbsp; I'm sure that one can come with a theory that violates this constraint, but I don't see much point in it.</p><p>If there were some means by which antimatter were coming into existence in our universe and interacting with normal matter in a big way, the result would be a lot of photons and an increase in mass of the universe -- antimatter has mass just like regular matter and that mass does not go away when it is annihilated by regular matter -- since matter and energy are but two sides of the same coin.&nbsp; This might not immediately result in the Big Crunch, but it would be exciting, particularly if the overlap included a sizeable chunk of the universe --&nbsp;say the part that we are in.</p><p>As derekmcd noted, annihilation of matter and antimatter produces ordinary energy not some sort of antienergy.<br /></p> <div class="Discussion_UserSignature"> </div>

 

[BetweenMyths]

<p>I came across this explanation regarding the universe being flat. "When&nbsp;it is said that the universe is flat, it is not in the same sense that a piece of paper is flat, but rather means that the geometry of the universe is such that parallel lines will never cross, the angles in a triangle will always add up to 180 degress, and the corners of cubes will always make right angles".</p><p>I've seen computer models of the observable universe, does anyone on the messageboard have a link to an&nbsp;artist's impression or computer model of what the <strong>entire flat universe</strong> probably looks like?</p>

 

[SpeedFreek]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I've seen computer models of the observable universe, does anyone on the messageboard have a link to an&nbsp;artist's impression or computer model of what the entire flat universe probably looks like? <br /> Posted by BetweenMyths</DIV></p><p>We have no idea what the size or shape of the entire universe is and so we cannot visualise it from the "outside", as it were. We can only visualise our observable universe and then extrapolate from that. Our observable universe is a sphere of space filled with galaxies, currently 46 billion light years in radius, with us at the centre. But it is only a sphere because the time light has had in which to travel is finite, and the light approaches us from all directions. </p> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

 

[derekmcd]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I came across this explanation regarding the universe being flat. "When&nbsp;it is said that the universe is flat, it is not in the same sense that a piece of paper is flat, but rather means that the geometry of the universe is such that parallel lines will never cross, the angles in a triangle will always add up to 180 degress, and the corners of cubes will always make right angles".I've seen computer models of the observable universe, does anyone on the messageboard have a link to an&nbsp;artist's impression or computer model of what the entire flat universe probably looks like? <br /> Posted by BetweenMyths</DIV></p><p>A "flat" universe is really just an abstract way of describing the 4 dimensional spacetime manifold.&nbsp; The 4 manifold we exist in is analogous to a 'surface' becuase it can show curvature in the presence of mass.&nbsp; Too much mass it is considered closed (represented by a sphere), just the right amount and it is considered flat (a flat plane), and not enough it is shown as saddle shaped.</p><p>&nbsp;</p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>

 

[bzannone]

&nbsp; If the expansion of the cosmos from a central event -ala Big Bang- is true, which is an easily enough understood scenario, and if it is evidenced by the distances of celestial bodies&nbsp; (ie- galaxies)&nbsp; increasing, how is it that galaxies collide? Furthermore, if one wants to consider some local systems to be within one fixed gravitational neighborhood (also simple enough to grasp) then why are rogue galaxies and/or black holes able to drift within these systems?

 

[UncertainH]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>If space is expanding, wouldn't our tools used to measure that expansion also be expanding, therefore rendering their measurement invalid? Could the existence or not of this phenomenon clarify the question of local versus global expansion?&nbsp;</p><p>Posted by vitalstatistic63</DIV></p><p>&nbsp;</p><p>That sounds logical and implies that both space and time expand in unison such that our laws of physics don't have to continually change to accomodate the expansion. However by the same logic, from our perspective the wavelength and frequency of the observed phtons that have passed through regions of expanding space should not appear to be redshifted because both it and our tools to measure it have changed in the same way. </p>

 

[derekmcd]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>&nbsp; If the expansion of the cosmos from a central event -ala Big Bang- is true, which is an easily enough understood scenario, and if it is evidenced by the distances of celestial bodies&nbsp; (ie- galaxies)&nbsp; increasing, how is it that galaxies collide? Furthermore, if one wants to consider some local systems to be within one fixed gravitational neighborhood (also simple enough to grasp) then why are rogue galaxies and/or black holes able to drift within these systems? <br /> Posted by bzannone</DIV></p><p>I see it all the time in articles that say galaxies are receding due to the expansion of space.&nbsp; It is really an incomplete statement.&nbsp; A more complete statement might be that large scale structrures of the observable universe are receding from each other.&nbsp; There is a somewhat of a hierarchy that goes from galaxies --> local groups --> clusters --> supercluster <--> filaments.&nbsp; </p><p>Galaxies and local groups are bound within the gravity wells of their cluster and are suseptible to local perterbations.&nbsp; Even at the scale of clusters, there are, on occastion, "collisions".&nbsp; Generally, at the scale of cluster, on average, they are receding due to the Hubble constant.&nbsp; It's really the superclusters that they are referring to when speaking of the Hubble constant and the metric expansion of space. </p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>

 

[derekmcd]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>&nbsp;That sounds logical and implies that both space and time expand in unison such that our laws of physics don't have to continually change to accomodate the expansion. However by the same logic, from our perspective the wavelength and frequency of the observed phtons that have passed through regions of expanding space should not appear to be redshifted because both it and our tools to measure it have changed in the same way. <br /> Posted by UncertainH</DIV></p><p>Technically, neither space nor time are expanding.&nbsp; It is the metric that is expanding.&nbsp; In other words, the distance between coordinates within the manifold are increasing.</p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>

 

[bzannone]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I see it all the time in articles that say galaxies are receding due to the expansion of space.&nbsp; It is really an incomplete statement.&nbsp; A more complete statement might be that large scale structrures of the observable universe are receding from each other.&nbsp; There is a somewhat of a hierarchy that goes from galaxies --> local groups --> clusters --> supercluster <--> filaments.&nbsp; Galaxies and local groups are bound within the gravity wells of their cluster and are suseptible to local perterbations.&nbsp; Even at the scale of clusters, there are, on occastion, "collisions".&nbsp; Generally, at the scale of cluster, on average, they are receding due to the Hubble constant.&nbsp; It's really the superclusters that they are referring to when speaking of the Hubble constant and the metric expansion of space. <br />Posted by derekmcd</DIV><br />&nbsp; </p><p>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Thank you for your reply. Let me see if I understand what you are saying. A super cluster would be a group of galaxies gravitationally bound together and galaxies contain stars and/or clusters bound within their respective systems. Cosmic expansion is the expanse of space (increase of distance/red shift) relative to superclusters. OK,&nbsp; I can see where that could be. So the collisions of galaxies&nbsp;and fact that Andromeda is 'drifting' closer to the Milky Way&nbsp;are events within the supercluster put into motion through some traumatic influence ie- force from the collapse of an unstable juvenile galaxy unable to maintain its structure, and the wandering rogue black hole would be an inter-galactic phenomenon caused by say a cluster of stars losing there planetary satellites&nbsp;thereby drifting off their own gravitational 'places' in the galactic framework creating a void for a neutron star or singularity to exploit and free itself of its position within the system, or any other similar cause/effect scenarios.&nbsp;Not the most solid explanation for these contradictions, but I could see some plausibility. Thank you again for your input.</p>

 

[R1]

<p><font size="2">Here is a link to a great class on video about the Big Bang and Expansion:</font></p><p><font size="2">&nbsp;http://www.space.com/common/community/forums/?plckForumPage=ForumDiscussion&plckDiscussionId=Cat%3ac7921f8b-94ec-454a-9715-3770aac6e2caForum%3ab1675c48-7c2d-4e4d-b6eb-8790beb68f8bDiscussion%3adcb2fa87-62c3-4561-a18f-b2c9b1e0682f&plckCategoryCurrentPage=0<br /></font></p> <div class="Discussion_UserSignature"> </div>