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Is the universe really expanding?

  1. Mar 12, 2014 #1
    Forgive me if this is stupid or has been covered many times in the past but I have a question about dark energy and the expansion of the universe. But first, consider a question posed by Poincare in the eighteenth century. He asked, “If we woke up one morning and everything in the universe had become a thousand times larger (including us and all our measuring devices), would there be any way for us to tell?” (of course the answer is no we couldn’t tell, because size is relative).

    Now consider the expansion of the fabric of the universe. Observations tell us that the expansion is real and our best theory to explain this stretching is by invoking a force called ‘dark energy’ that accounts for some or all of the energy required to make this stretching happen. But currently we cannot detect this energy and can merely infer its existence through mathematical modelling.

    But what if our observations are misleading us. Is it possible that instead of inter-stellar space stretching, it is actually our local region of space, along with all the other mass rich regions, that are in fact contracting? This would produce the illusion of inter-stellar space stretching without needing dark energy to achieve it.

    I can’t see how to proved this local shrinkage since, as Poincare points out, all our measuring devices would shrink at the same rate too. I just wondered if local space contraction could have played tricks with our observations and lead us to hunt for an explanation involving dark energy that is not needed?
  2. jcsd
  3. Mar 12, 2014 #2
    Be careful of philosophy here, it doesn't fly well. :-)

    We do know the universe is expanding. Most simply, it is by observation of distant objects in "red shift". Redshift is caused by the expansion of the fabric of the space - the expansion of the 'graph paper' on which the Universe is drawn. Doppler Shift is caused by the relative motion of the source and the observer.

    From this website: http://skolor.nacka.se/samskolan/eaae/summerschools/Hubble.html [Broken]

    If I'm not mistaken, Einstein's Cosmological Constant was an effort by Einstein to explain a steady-state universe because his theories of relativity predicted an expanding universe. I just got done reading a paper by a guy (the same one that co-wrote the Planck Star paper) that "dark energy" could possibly be seen as Einstein's theories re-proven.
    Last edited by a moderator: May 6, 2017
  4. Mar 12, 2014 #3


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    This topic has been beat to death here. Try a forum search. Changing the scale creates problems.

    As D english said, this forum is not the place for your personal speculation. Come at things on this forum from the point of view that "I think modern science is wrong and here's why" but rather "I'm sure modern science is right, but I can't quite follow the logic on ... so please help my understand why it is true "

    There are other forums on the internet for speculation outside of accepted physics
  5. Mar 12, 2014 #4


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    All you have done is replace one problem with a new one - you haven't solved any: what causes the shrinking?
  6. Mar 12, 2014 #5
    Has anyone seen this paper? Its from 2010, wondering what opinions are about it:


    The expansion of the observed universe appears to be accelerating. A simple explanation of this
    phenomenon is provided by the non-vanishing of the cosmological constant in the Einstein equations.
    Arguments are commonly presented to the effect that this simple explanation is not viable or not
    sufficient, and therefore we are facing the "great mystery" of the "nature of a dark energy". We
    argue that these arguments are unconvincing, or ill-founded.
  7. Mar 12, 2014 #6
    Believe it or not they’re just the sort of answers I was hoping for. My physics background is at the popular science book level so when I think of a specific problem like this it’s quite often the case that the on-line explanations go over my head. I knew the evidence for the expansion came from red-shift disparities between the light from near and distant supernovae but I couldn’t work out whether shrinking the scale at the observer end could give a red-shift ‘look’ to the results. If you say changing the scale causes problems then I guess that’s my answer (although it’s kind of reassuring to know that someone else must have thought to check, which means my question wasn’t completely dum).

    Thanks guys.
  8. Mar 12, 2014 #7


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    There are no dumb questions, but you might give some consideration to the fact that there are thousands upon thousands of people who have spent years going over this stuff and it really is pretty unlikely that you or I are going to come up with some notion like this and find that none of them have thought of it.

    When you think of something that seems to go against established physics, here's some good advice that I have given before.

    When you come up against something that flies so utterly in the face of established science, it is not a good idea to start off reaching different conclusions and stating them as correct but rather to start off with the assumption that you have made a mistake somewhere and try to find out where it is. If you have NOT made a mistake you will find the flaw in the established science, but that is very unlikely to happen. If you start off thinking that you have overturned established science you are likely to just end up embarrassed.
  9. Mar 12, 2014 #8
    From a guy that spent more time in the military than the math class, you are not dumb. You just need to learn the protocol. Welcome to the club, don't be ashamed.

    I'm pretty sure that "shrinking the scale at the the observer end" would be describing doppler effect, not red shift. I'm hoping a more educated person than me will confirm/correct.
  10. Mar 12, 2014 #9

    No in cosmology their are three distinct shifts.
    Doppler shift is motion, gravitational redshift is due to gravity effects. Cosmological redshift is due to expansion.
    any of the above can potentially redshift or blueshift
    For example Andromeda is blueshifted due to its doppler shift as it is moving towards us.
    As light enters a gravity well it blueshifts when it exits it then redshifts the same amount unless there is enough time for a measurable change in the orginal gravitational background to change. Google Sache-wolfe effect. If the universe contracts objects will blueshift however this doesnt occur so we see primarily cosmological redshift.

    Each act the same as doppler effect however the causes are distinctive. Therefore its best to the three types distinctive in terminology.
    Last edited: Mar 12, 2014
  11. Mar 12, 2014 #10
    This article will provide better details

    1) What is outside the universe?
    2) What is causing the expansion of the universe?
    3) Is expansion, faster than light in parts of the Universe, and How does this not violate the faster than light speed limit?
    4) What do we mean when an object leaves our universe?
    5) What do we mean when we say homogeneous and isotropic?
    6) Why is the CMB so vital in cosmology?
    7) Why is the LambdaCDM so vital to cosmologists?
    8) Why are all the galaxies accelerating from us?
    9) Is Redshift the same as Doppler shift?
    9) How do we measure the distance to galaxies?
    10) What is a Cepheid or standard candle

    These are some of the common questions I will attempt to address in the following article
    First we must define some terms and symbols used.

    Planck constant: [itex]h\ =\ 6.62606876(52)\ \times\ 10^{-34}\ J\ s[/itex]
    Gravitational constant: [itex]G\ =\ 6.673(10)\ \times\ 10^{-11}\ m^{3} kg^{-1} s^{-2}[/itex]
    Speed of light in a vacuum:[itex]c\ =\ 2.99792458\ \times\ 10^{8}\ m\ s^{-1}[/itex]

    The parsec (symbol: pc) is a unit of length used in astronomy, equal to about 30.9 trillion kilometers (19.2 trillion miles). In astronomical terms, it is equal to 3.26 light-years, and in scientific terms it is equal to 3.09×1013 kilometers
    Mpc=1 million Parsecs

    Universe: A generalized definition of the universe can be described as everything that is. In Cosmology the universe can be described as everything measurable in our space-time either directly or indirectly. This definition forms the basis of the observable universe. The Hot Big Bang model does not describe prior to 10-43 seconds. The LambdaCDM or [itex]\Lambda[/itex]CDM model is a fine tuned version of the general FLRW (Freidmann Lemaitre Robertson Walker) metrics, where the six observationally based model parameters are chosen for the best fit to our universe.

    The Observable universe is 46 Billion light years, or 4.3×1026 meters with an age as of 2013, is 13.772 ± 0.059 billion years.
    In the hot big bang model we do not think of the universe as starting from a singularity (infinitely, hot, dense point) instead measurements agree space-time as simply expanding. That expansion is homogeneous and isotropic. If you were to take a telescope and look at the night sky, no matter where you look the universe looks the same or homogeneous meaning no preferred location. As you change directions with the telescope you will find that no matter which direction you look the universe looks the same or isotropic meaning no preferred direction. These terms in cosmology are only accurate at certain scales. Below 100Mpc it is obvious that the universe is inhomogeneous and anisotropic. As such objects as stars and galaxies reside in this scale. This also tells us that there is no center of the universe, as a center is a preferred location. These terms also describe expansion. Expansion will be covered in more detail in the Cosmological Redshift section. Whether or not the universe is finite or infinite is not known. However if it is infinite now so it must be in the beginning.
    Common misconceptions arise when one tries to visualize a finite universe such questions include.

    "So how do we see farther than 13.772 billion light years?" The answer lies in expansion; as light is travelling towards us, space-time has expanded.
    “If the universe is finite what exists outside the Universe?" If you think about this question with the above definition of the universe you will realize that the question is meaningless. One accurate answer in regards to cosmology is nonexistent.
    "What makes up the barrier between our universe and outside our universe?" The short answer is there is no barrier.

    The CMB, (Cosmic Microwave Background) The CMB is thermal radiation filling the Observable universe almost uniformly, This provides strong evidence of the homogeneous and isotropic measurements and distances. As the universe expanded, both the plasma and the radiation filling it grew cooler. When the universe cooled enough, protons and electrons combined to form neutral atoms. These atoms could no longer absorb the thermal radiation, and so the universe became transparent instead of being an opaque fog. Precise measurements of cosmic background radiation are critical to cosmology, since any proposed model of the universe must explain this radiation. CMB photons were emitted at about 3000 Kelvin and are now 2.73 Kelvin blackbody radiation. Their currently observed energy is 1/1000th of their energy as emitted.

    In order to measure an objects motion and distance in cosmology it is important to properly understand redshift, Doppler shift and gravitational redshift. Incorrect usage of any of these can lead to errors in our measurements.

    Doppler shift and redshift are the same phenomenon in general relativity. However you will often see Doppler factored into components with different names used, as will be explained below. In all cases of Doppler, the light emitted by one body and received by the other will be red or blueshifted i.e. its wavelength will be stretched. So the color of the light is more towards the red or blue end of the spectrum. As shown by the formula below.

    [tex]\frac{\Delta_f}{f} = \frac{\lambda}{\lambda_o} = \frac{v}{c}=\frac{E_o}{E}=\frac{hc}{\lambda_o} \frac{\lambda}{hc}[/tex]

    The Cosmological Redshift is a redshift attributed to the expansion of space. The expansion causes a Recession Velocity for galaxies (on average) that is proportional to DISTANCE.
    A key note is expansion is the same throughout the cosmos. However gravity in galaxy clusters is strong enough to prevent expansion. In other words galaxy clusters are gravitationally bound. In regards to expansion it is important to realize that galaxies are not moving from us due to inertia, rather the space between two coordinates are expanding. One way to visualize this is to use a grid where each vertical and horizontal joint is a coordinate. The space between the coordinates increase rather than the coordinates changing. This is important in that no FORCE is acting upon the galaxies to cause expansion. As expansion is homogeneous and isotropic then there is no difference in expansion at one location or another. In the [itex]\Lambda[/itex]CDM model expansion is attributed to the cosmological constant described later on. The rate a galaxy is moving from us is referred to as recession velocity. This recession velocity then produces a Doppler (red) shift proportional to distance (please note that this recession velocity must be converted to a relative velocity along the light path before it can be used in the Doppler formula). The further away an object is the greater the amount of redshift. This is given in accordance with Hubble’s Law. In order to quantify the velocity of this galactic movement, Hubble proposed Hubble's Law of Cosmic Expansion, aka Hubble's law, an equation that states:

    Hubble’s Law: The greater the distance of measurement the greater the recessive velocity

    Velocity = H0 × distance.

    Velocity represents the galaxy's recessive velocity; H0 is the Hubble constant, or parameter that indicates the rate at which the universe is expanding; and distance is the galaxy's distance from the one with which it's being compared.

    The Hubble Constant The Hubble “constant” is a constant only in space, not in time,the subscript ‘0’ indicates the value of the Hubble constant today and the Hubble parameter is thought to be decreasing with time. The current accepted value is 70 kilometers/second per mega parsec, or Mpc. The latter being a unit of distance in intergalactic space described above.
    Any measurement of redshift above the Hubble distance defined as H0 = 4300±400 Mpc will have a recessive velocity of greater than the speed of light. This does not violate GR because a recession velocity is not a relative velocity or an inertial velocity. It is precisely analogous to a separation speed. If, in one frame of reference, one object is moving east at .9c, and another west at .9c, they are separating by 1.8c. This is their recession velocity. Their relative velocity remains less than c. In cosmology, two things change from this simple picture: expansion can cause separation speeds much greater even than 2c; and relative velocity is not unique, but no matter what path it is compared along, it is always less than c, as expected.

    z = (Observed wavelength - Rest wavelength)/(Rest wavelength) or more accurately

    1+z= λobservedemitted or z=(λobservedemitted)/λemitted

    [tex]1+Z=\frac{\lambda}{\lambda_o}[/tex] or [tex]1+Z=\frac{\lambda-\lambda_o}{\lambda_o}[/tex]

    λ0= rest wavelength
    Note that positive values of z correspond to increased wavelengths (redshifts).
    Strictly speaking, when z < 0, this quantity is called a blueshift, rather than
    a redshift. However, the vast majority of galaxies have z > 0. One notable blueshift example is the Andromeda Galaxy, which is gravitationally bound and approaching the Milky Way.
    WMAP nine-year results give the redshift of photon decoupling as z=1091.64 ± 0.47 So if the matter that originally emitted the oldest CMBR photons has a present distance of 46 billion light years, then at the time of decoupling when the photons were originally emitted, the distance would have been only about 42 million light-years away.

    Cosmological Constant is a homogeneous energy density that causes the expansion of the universe to accelerate. Originally proposed early in the development of general relativity in order to allow a static universe solution it was subsequently abandoned when the universe was found to be expanding. Now the cosmological constant is invoked to explain the observed acceleration of the expansion of the universe. The cosmological constant is the simplest realization of dark energy, which the more generic name is given to the unknown cause of the acceleration of the universe. Indeed what we term as "Dark" energy is an unknown energy that comprises most of the energy density of our cosmos around 73%. However the amount of dark energy per m3 is quite small. Some estimates are around about 6 × 10-10 joules per cubic meter. However their is a lot of space between large scale clusters, so that small amount per m3 adds up to a significant amount of energy in total. In the De_Sitter FLRW metric (matter removed model)
    this is described in the form.


    Another term often used for the cosmological constant is vacuum energy described originally by the false vacuum inflationary Model by A.Guth. The cosmological constant uses the symbol Λ, the Greek letter Lambda.
    The dark energy density parameter is given in the form:
    [itex]\Omega_\Lambda[/itex] which is approximately 0.685

    The Doppler Redshift results from the relative motion of the light emitting object and the observer. If the source of light is moving away from you then the wavelength of the light is stretched out, i.e., the light is shifted towards the red. When the wavelength is compressed from an object moving towards you then it moves towards the blue end of the spectrum. These effects, individually called the blueshift and the redshift are together known as Doppler shifts. The shift in the wavelength is given by a simple formula

    (Observed wavelength - Rest wavelength)/(Rest wavelength) = (v/c)

    [tex] f=\frac{c+v_r}{c+v_s}f_o[/tex]

    c=velocity of waves in a medium
    [tex]v_r[/tex] is the velocity measured by the source using the source’s own proper-time clock(positive if moving toward the source
    [tex]v_s[/tex] is the velocity measured by the receiver using the source’s own proper-time clock(positive if moving away from the receiver)

    The above are for velocities where the source is directly away or towards the observer and for low velocities less than relativistic velocities. A relativistic Doppler formula is required when velocity is comparable to the speed of light. There are different variations of the above formula for transverse Doppler shift or other angles. Doppler shift is used to describe redshift due to inertial velocity one example is a car moving away from you the light will be redshifted, as it approaches you the light and sound will be blueshifted. In general relativity and cosmology, there is a fundamental complication in this simple picture - relative velocity cannot be defined uniquely over large distances. However, it does become unique when compared along the path of light. With relative velocity compared along the path of the light, the special relativity Doppler formula describes redshift for all situations in general relativity and cosmology. It is important to realize that gravity and expansion of the universe affect light paths, and how emitter velocity information is carried along a light path; thus gravity and expansion contribute to Doppler redshift

    Gravitational Redshift describes Doppler between static emitter and receiver in a gravitational field. Static observers in a gravitational field are accelerating, not inertial, in general relativity. As a result (even though they are static) they have a relative velocity in the sense described under Doppler. Because they are static, so is this relative velocity along a light path. In fact, the relative velocity for Doppler turns out to depend only on the difference in gravitational potential between their positions. Typically, we dispense with discussion of the relative velocity along a light path for static observers, and directly describe the resulting redshift as a function of potential difference. When the potential increases from emitter to receiver, you have redshift; when it decreases you have blue shift. The formula below is the gravitational redshift formula or Einstein shift off the vacuum surrounding an uncharged, non rotating, spherical mass.
    \frac{\lambda}{\lambda_o}=\frac{1}{\sqrt{(1 - \frac{2GM}{r c^2})}}

    G=gravitational constant
    c=speed of light
    M=mass of gravitational body
    r= the radial coordinate (measured as the circumference, divided by 2pi, of a sphere centered around the massive body)

    The rate of expansion is expressed in the [itex]\Lambda[/itex]CDM model in terms of
    The scale factor, cosmic scale factor or sometimes the Robertson-Walker scale factor parameter of the Friedmann equations represents the relative expansion of the universe. It relates the proper distance which can change over time, or the comoving distance which is the distance at a given reference in time.


    where d(t) is the proper distance at epoch (t)
    d0 is the distance at the reference time (to)
    a(t) is the comoving angular scale factor. Which is the distance coordinate for calculating proper distance between objects at the same epoch (time)
    r(t) is the comoving radial scale factor. Which is distance coordinates for calculating proper distances between objects at two different epochs (time)

    [tex]Proper distance =\frac{\stackrel{.}{a}(t)}{a}[/tex]

    The dot above a indicates change in.

    the notation R(t) indicates that the scale factor is a function of time and its value changes with time. R(t)<1 is the past, R(t)=1 is the present and R(t)>1 is the future.


    Expansion velocity
    [tex] v=\frac{\stackrel{.}{a}(t)}{a}[/tex]

    This shows that Hubble's constant is time dependant.

    Cosmic Distance ladder, also known as Extragalactic distance scale. Is easily thought of as a series of different measurement methods for specific distance scales. Previous in the article we discussed the various forms of Redshift. These principles are used in conjunction with the following methods described below. Modern equipment now allows use spectrometry. Spectrographs of an element give off a definite spectrum of light or wavelengths. By examining changes in this spectrum and other electromagnetic frequencies with the various forms of shifts caused by relative motion, gravitational effects and expansion. We can now judge an objects luminosity where absolute luminosity is the amount of energy emitted per second.

    Luminosity is often measured in flux where flux is

    [tex]f=\frac{L}{4\pi r^2}[/tex]

    However cosmologists typically use a scale called magnitudes. The magnitude scale has been developed so that a 5 magnitude change corresponds to a differents of 100 flux.
    Rather than cover a large range of those distance scales or rungs on the ladder I will cover a few of the essential steps to cosmological distance scales. The first rung on the ladder is naturally.

    Direct measurements: Direct measurements form the fundamental distance scale. Units such as the distance from Earth to the sun that are used to develop a fundamental unit called astronomical unit or AU. During the orbit around the sun we can take a variety of measurements such as Doppler shifts to use as a calibration for the AU unit. This Unit is also derived by a method called Parallax.

    Parallax. Parallax is essentially trigonometric measurements of a nearby object in space. When our orbit forms a right angle triangle to us and the object to be measured
    With the standardized AU unit we can take two AU to form the short leg. With the Sun at a right angle to us the distance to the object to be measured is the long leg of the triangle.

    Moving Cluster Parallax is a technique where the motions of individual stars in a nearby star cluster can be used to find the distance to the cluster.

    Stellar parallax is the effect of parallax on distant stars . It is parallax on an interstellar scale, and allows us to set a standard for the parsec.

    Standard candles A common misconception of standard candles is that only type 1A supernova are used. Indeed any known fundamental distance measurement or stellar object whose luminosity or brightness is known can be used as a standard candle. By comparing an objects luminosity to the observed brightness we can calculate the distance to an object using the inverse square law. Standard candles include any object of known luminosity, such as Cepheid’s, novae, Type 1A supernova and galaxy clusters.

    My thanks to the following Contributors, for their feedback and support.

    Jonathon Scott

    Article by Mordred, PAllen
  12. Mar 13, 2014 #11
    Thanks for posting that Mordred, I read all of it. Its easy to understand, and easy to read.
  13. Mar 14, 2014 #12
    Really nice posting Mordred. I get slightly lost on the gravitational redshift section but at least I now have a better understanding of how and when to apply these different flavours of redshift.

    Can I just ask a couple of supplementary questions related to expansion. I’m afraid they’re still at a very simplified level and I understand that reality is much more messy than this idealised example but at least this time I’ve tried to phrase the questions properly instead of using vague assertions. I’m puzzled by the relationship between matter and space. I read on another thread that the force of expansion is universal but is so weak that in matter-rich regions of space the expansion is arrested by the normal 4 forces of matter. I’m just wondering how this manifests itself. For example, if you have a single galaxy travelling through space it will temporarily halt the local expansion of space as it passes. Lets say it takes a billion years from when a point in space encounters the leading edge of the galaxy to it clearing the trailing edge. So the fabric of space at the front edge has had a billion years worth of expansion more than the fabric at the back edge.

    1) Does that have any measurable effect on the matter in the galaxy or on light travelling through it?

    And presumably the galaxy produces a kind of wake through space, the centre of which will have received the full billion years worth of non-expansion. So

    2) Would this have an effect on light travelling across the wake? (or am I taking the whole fabric of space thing too literally?)
  14. Mar 14, 2014 #13


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    For one thing, I think you're taking "fabric" too far. "Fabric" is not a good analogy for spacetime. Too many problems.

    What you seem to have lost sight of is that the trailing edge of your moving galaxy moves just as much as the leading edge, so at no time does the galaxy occupy any more of space than at any other time.

    Also, Google "metric expansion" which may give you a bit simpler explanation. Space doesn't really move, things just get farther apart.
  15. Mar 14, 2014 #14


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    what about transverse redshift?
  16. Mar 14, 2014 #15
    Yes, I feared that might be the case

    We may be at cross purposes here. What I meant was there will be a small region of space at the trailing edge of the galaxy that has undergone no expansion throughout the last billion years because the galaxy has been passing over it. At the same time the leading edge is encountering 'free space' that hasn't had its expansion halted over the last billion years because it was yet to come under this galaxies region of influence.
  17. Mar 17, 2014 #16

    Tranverse redshift is a specilized redshift formula. Details and formula is covered here


    thanks to everyone on the feedback on the article. I had good PF members help on its writing.
  18. Mar 20, 2014 #17
    Yes that's a helpful article for understanding the universe.

    Astrophysics scientists are amazing, they could explain a lot by just analyzing the properties of the photons that came from the cosmos... Galaxies in local clusters sharing gravitational influence do not participate in the expansion of the universe... and each cluster is basically stationary, the apparent receding of other cluster or galaxies is due to the expansion of the spacetime itself. The Olbers' Paradox suggests too that the universe is expanding, I guess it could be proved with mathematics. http://math.ucr.edu/home/baez/physics/Relativity/GR/olbers.html

    Btw, the speed radar being used by traffic enforcers operates on the principle of doppler shift. Some protested their speeding tickets and contested that the speed radar used on them are not accurate, which make me think about the accuracy of doppler shift measurement in astronomy. And I'm guessing that the distance measurement ladder is somehow being used to make sure that the doppler shift measurement is accurate?
  19. Mar 20, 2014 #18


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    Again, you are carrying "fabric" too far. There was no THING there as the galaxy passed through a region. It didn't displace anything. You need to drop this idea of space as a thing / fabric and just consider material objects and their positions relative to each other. Again, Google "metric expansion"
  20. Mar 20, 2014 #19
    Thank you for the excellent and informative article. I have been thinking about the expansion of the universe for a few years, and I have some questions relating to a few of the points in the article:

    1. What are the actual definitions of isotropic and homogenous? When we say "the same in all directions" or "there is no location which is 'special' in the universe", am I correct in understanding that this only applies to the three spatial dimensions in terms of direction/location?

    2. Is it (theoretically) possible for our local region of the universe in our reference frame to actually have been around (i.e. filled with mostly space rather than plasma) for longer than 13.8 bn years, for example, if the expansion was slower initially, but has been accelerating since, or is there something fundamental that rules this out?
  21. Mar 20, 2014 #20


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    Are there any OTHER spatial dimensions? I don't really understand your question.

    It is fundamenttally ruled out by virtue of the fact that the universe is not older than that so NOTHING is older than that.
    Last edited: Mar 20, 2014
  22. Mar 20, 2014 #21
    I'm slightly annoyed with myself that I couldn't quite make you see what I meant. I know this because you've used the word 'displace' in your answer. I think this arose because I used the word 'wake' to describe the space through which the galaxy has travelled. It was not my intention to suggest space is moved aside. However, I'm getting the more general message that thinking of space as a 'fabric-like' entity will only lead to problems.

    I do have some other stuff I'd like to ask on the same general subject but seeing as that would move discussion further away from the origin question of this thread, I'll probably start a new thread some time.

    Many thanks all
  23. Mar 20, 2014 #22
    If he's wondering what I'm thinking he might be wondering, this question is difficult to state, but it is one that every thinker is going to ask when seeing "isotropic and homogeneous - the same in all directions"...

    Since it is assumed that viewing distant regions is looking into the past, the attributes one observes like density (of galaxies, or galactic clusters, or super clusters, etc...) must be interpreted as their density in the past.

    This raises the question, "What is the same"? If the observed density of a distant region is "the same" as locally, then the two densities are only the same at their different respective elapsed times of the expansion. One would infer that the distant region "now" must be more expanded than observed, so not "the same" for a particular "now". (I know GR won't extend IFRs indefinitely in the presence of gravity; maybe this part of the question can be reformulated to still point out the discrepancy with time change of density in a more GR friendly way).

    Or, the observed distant region could present a different density than local, but with an adjustment for the time lapse and the rate of expansion, a corrected value is found to match local... which also does not meet the spirit of "the same".

    So back to "What is the same?", and what is actually observed, and how do astronomers actually mean "the same"?
  24. Mar 20, 2014 #23


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    The 'shrinking' vs 'expanding' universe poses questions that should reveal which is a better fit. For example, is the ratio of galaxy size to distance constant over billions of years? In a shrinking universe, the answer is yes. In the observed universe, not so much? The same line of reasoning applies to the solar system. Have planetary distances from the sun increased over the past 4 billion years?
  25. Mar 21, 2014 #24
    Can dark energy be centripetal force?

    I don't want to start a whole thread about this because I think this idea can be stupid, so I decided to ask here.

    I know that someone else has considered this possibility, but I at least want to know why it can't be true.
    Can dark energy be centripetal force?
    My theory is that the Universe could be spinning, and that's what pushing galaxies apart. When we discovered the acceleration in the expansion of the Universe, we measured the redshift of one supernova in one spot of the sky, right? Perhaps if we do the same experiment but in a point in the sky that is about 90º from the previous one, we can find out that the Universe isn't accelerating in that point, because that would be the axis of rotation.

    Considering that nowadays many physicists believe in the Multiverse theory, it makes sense to say that our entire Universe is moving and/or spinning. It is just one in a sea of "bubbles".

    Tell me why I am wrong.
  26. Mar 21, 2014 #25
    Sorry, I realise I was unclear in my original post. What I meant was:

    1. I was referring to the time dimension, which I believe from Special Relativity is difficult to separate from the three spatial dimensions due to the lack of absolute time, and the fact that each separate observer has his own notion of distances and time.

    2. I had understood (although please correct me if I am wrong) that the 'age of the universe' was determined by considering the Hubble constant, and at what time in the past all matter would have been located at one point (or at least very close together) based on a constant rate of expansion (constant in terms of relative velocities of galaxies rather than a constant percentage expansion). Instead of assuming a constant rate of expansion, if I theoretically assume an accelerating expansion, would this result in an older universe? Or is there something fundamental that would contradict this? For example, if astronomers believed that most stars would have burnt out by 15bn years but we do not observe this in practice, I could rule out with some confidence that the universe is likely to be over 15bn years old.
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