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Dark energy acceleration of space effect on red shift

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  1. Jul 16, 2013 #1
    I have not seen this discussed so this a question in the form of a statement for clarity.

    If light left a star 10 billion years ago and space has expanded for 10 billion years before the light is observed the light frequency would be stretched out or red shifted therefore some of the red shift observed would be due to distance over time. The velocity of light is constant but its speed is dependent on the gravitational field the light is in and; black holes can eject ultra high frequency gamma rays. Dark energy functions as antigravity so shifts light to lower frequency over time and space.

    This is only a logic question and logic creates consistency but not accuracy if the data is misinterpreted or just wrong. Considering the relatively new discovery of universal accelerating expansion of the cosmos, has the effects on red shift been questioned? If not then why not?

    This is just a question I asked myself and wanted some feedback.

    Thanks for any help.
     
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  3. Jul 16, 2013 #2

    bapowell

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    The redshifting of light is not specific to accelerated expansion. Expansion in general of any kind will cause a redshift, because the wavelength of light is proportional to the scale factor, [itex]\lambda \propto a(t)[/itex] (the scale factor governs the increase in distance in the universe, so a(t) = constant would be a static universe).

    I'm confused about your distinction between the velocity and speed of light. The speed of light in the universe is invariant -- it has the value [itex]c[/itex] for all observers. This is not related to light being trapped by black holes, which I think you might be supposing?

    Regarding the ejection of radiation from black holes, that process actually occurs outside the black hole, in the vicinity of the accretion disc. The accretion disc consists of material that is superheated by the compression caused by the black hole's gravity.
     
  4. Jul 16, 2013 #3
    Can you state that in a ordinary logical narrative. I person should not need more than logic and narrative language for understanding. A person who wants to work in the field needs tools of the trade. I find it is useful to keep context constancy for mutual understanding.

    I was "schooled" by a moderator member of a physics forum that the speed of light is a misnomer the correct statement is the velocity of light is fixed but speed varies due to medium - it could be water or gravity field.

    I do know and understand the mechanics you describe. I know how you are right. I was just using diametric logic as framing. Dark energy (anti-gravity) expands space and red shifts light and its diametric "Gravity" decreases space and blue shifts light. Of course the mechanics of matter being confined to a smaller space with a increase in kinetic energy can result in gamma burst. (I am not making a red-shift claim here; just a logic framing.)

    I accept that to have a logical explanation on a cosmic scale that a diametric balance is required on a cosmic scale. Add up the diametric and it equals zero. Every equation needs to be balance to also be logical.

    A person should not need detail to see the big picture. A person can understand the idea of a forest without the details on each type of tree in it.
     
  5. Jul 16, 2013 #4
    The redshift properties due to expansion is constantly questioned, such questions that have to be answered on a particular reading comprises the following.

    1) what is the emitted wavelength, we use standard candles with known wavelength properties.
    2) is there any gravity wells/lenses etc influencing via gravitational redshift. For this we need numerous comparison measurements, refraction effects ie distortions as well as other methods too lengthy to go into to isolate.
    3) what is the apparent magnitude and luminosity of the emitting body. Both of these have a redshift relation and knowing the luminosity and magnitude of an emitting body can help confirm the distance measurement via the related formula's
    4) what is the doppler shift effects due to motion of emmiter and observer. This is trickier as it requires precision measurements, comparison data over a history of measurements to determine. Methods include various form of parallex (trigonometry math).

    Needless to say no method works in every circumstance, forms of parralex works well at some ranges but not others. For any distance measurement made, other confirmation methods are used to confirm that measurement. Such as parralex, database of results from other nearby and known objects, luminosity to distance relationsa, and apparent magnitude to redshift relations.

    Hope this helps my signature has various articles and tools to better understand redshift. the Cosmocalculator is particularly handy as it will graph as well as construct charts. You can also select what types of measurements you wish to display. It uses either WMAP or Planck data values typically flat geometry however you can play a bit with curvature effects as well.

    The other link will provide numerous articles on expansion and redshift as well as geometry.
     
  6. Jul 16, 2013 #5

    bapowell

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    Space expands so length-scales increase with time. Light travels through space. Light has a characteristic length scale (wavelength). Wavelength increases with time.
     
  7. Jul 16, 2013 #6
    this article is handy on distance measurements in a basic understanding. Its getting hard to find though so I will repost it.

    EXPANSION AND REDSHIFT
    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.

    Ho[itex]\propto\sqrt\Lambda[/itex]

    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.
    [tex]
    \frac{\lambda}{\lambda_o}=\frac{1}{\sqrt{(1 - \frac{2GM}{r c^2})}}
    [/tex]

    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.

    d(t)=a(t)do

    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.

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

    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.

    PAllen
    Naty1
    Jonathon Scott
    marcus

    Article by Mordred, PAllen
     
  8. Jul 16, 2013 #7
    here is a quick sampler of the cosmocalc found at

    http://www.einsteins-theory-of-relativity-4engineers.com/LightCone7/LightCone.html

    values are default other than column selection tough I did not include all the columns

    [tex]{\small\begin{array}{|c|c|c|c|c|c|}\hline R_{0} (Gly) & R_{\infty} (Gly) & S_{eq} & H_{0} & \Omega_\Lambda & \Omega_m\\ \hline 14.4&17.3&3400&67.9&0.693&0.307\\ \hline \end{array}}[/tex] [tex]{\small\begin{array}{|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|} \hline a=1/S&z&T (Gy)&R (Gly)&D_{now} (Gly)&D_{then}(Gly) \\ \hline 0.000&1089.000&0.0004&0.0006&45.332&0.042\\ \hline 0.003&338.773&0.0025&0.0040&44.184&0.130\\ \hline 0.009&104.913&0.0153&0.0235&42.012&0.397\\ \hline 0.030&32.015&0.0902&0.1363&38.052&1.153\\ \hline 0.097&9.291&0.5223&0.7851&30.918&3.004\\ \hline 0.312&2.208&2.9777&4.3736&18.248&5.688\\ \hline 1.000&0.000&13.7872&14.3999&0.000&0.000\\ \hline 3.208&-0.688&32.8849&17.1849&11.118&35.666\\ \hline 7.580&-0.868&47.7251&17.2911&14.219&107.786\\ \hline 17.911&-0.944&62.5981&17.2993&15.536&278.256\\ \hline 42.321&-0.976&77.4737&17.2998&16.093&681.061\\ \hline 100.000&-0.990&92.3494&17.2999&16.328&1632.838\\ \hline \end{array}}[/tex]
     
  9. Jul 18, 2013 #8
    Thank you, that was a refreshing read on redshift and I learned more about the topic. Breaking the topic into classifications of methods for measuring redshift I found maintained my focus - it is a keeper.
     
  10. Jul 18, 2013 #9
    I understand what you are saying. I need to express myself better and that is my problem to work on. I appreciate your patience in answering a simple question that I created unintentionally. Space is expanding and that causes redshift - and the amount of dark energy within space is increasing creating acceleration of expansion. Dark energy increasing over time may effect redshift. Since physics can not account for Dark energy increasing then all current physics in regard to red-shift caused by increasing Dark energy over time is blind to Dark energy effect on redshift - if any.

    There may be cosmological aspects of physics that can not be detected on a solar or human scale. Physics knows nothing scientifically about Dark matter or energy; so 95% of the universe is "Dark" or unknown to science. We have been able to detect Dark physics but not account for it through scientific explanation.

    I think science can only speculate about Dark cosmology at this point so it is a legitimate area of discussion that may not be an agreeable topic in this forum since teaching what is known is the goal here - I think.

    Can a speculative topic based in logical process and propositions be discussed in the context cosmology? I hope some fresh perspectives could simulate ideas that could be helpful for modeling by mathematical Cosmologist. Since Dark energy acts on space but does not interact with electromagnetism a logical explanation my be beyond reach.
     
  11. Jul 18, 2013 #10
    I'm glad you found the article useful, I had some excellent help in writing it.
     
  12. Jul 18, 2013 #11

    There is in fact a lot we do know about dark energy and dark matter, even though they are completely unrelated.

    Dark energy isn't mysterious in and of itself, fundamentally it is simply a vacuum energy. One key aspect in understanding current cosmology is recognizing its fluid nature. In terms of pressure. Visible matter or baryonic matter is a small contributor to the overall pressure. The majority of positive pressure is due to dark matter, as well as radiation (photons, neutrinos both relativistic and non relativistic). The negative pressure or dark energy is constant in energy density per m3. the overall total energy mass due to dark energy grows because the amount of available space has grown. The mystery is how dark energy stays constant as space increases. Popular mechanisms involve quantum field theory ie virtual particle interactions.

    Dark matter or non baryonic matter is as you stated weakly interactive, if it involves any other force other than gravity, we have not been able to measure that interaction. We know its there due to its indirect influences such as gravitational lensing and galaxy rotation. Its also a particle that due to lack of direct measurements we have not been able to fit into the standard model of particles. However there are detection tests underway to try and fill in our gaps on dark matter. Even though there are findings, caution and supportive material of those findings prevent them from being conclusive for now.
    So take heart our knowledge of the dark sector is continually improving.

    edit: forgot to add dark energy and dark matter are no longer considered speculative as per ce. It's the attempts at speculative theories involved without adequate supportive material that generates a need for moderation. One aspect I truly appreciate on this site is the desire for correct and main stay answers.
     
    Last edited: Jul 19, 2013
  13. Jul 19, 2013 #12
    Is dark energy constant in energy density? When cosmologist where informed 15 years ago that the universe expansion rate is accelerating some pointed out that required additional dark energy entering universe which had no explanation thru current scientific thinking. It was Alex Filippenko of Berkeley who I heard say that there is no physics to account for the acceleration. I saw it as an epistemological non sequitur - an effect with no logical cause; which I am amused to say, is the definition of magic. That is not the answer so new assumptions and logical framework are called for; and it isn't first time that has happened; the last time was quantum mechanics. If physics added quantum mechanics to accommodate data from the realm of the very small we may face the same type of dilemma to accommodate data from the very large like the acceleration data. In discussion of such topics correct logic is required but there could be no requirement of "main stay" answers because I think none exist at this point. I would like your opinion of this paragraph.

    Recognizing cosmology's fluid nature is a helpful analogy; thanks for that. I was using wave frequency probability dynamics which is harder to see and understand than fluid dynamics. Why complicate things when a less convoluted explanation is available. I am for simplicity when it works as you showed.
     
  14. Jul 19, 2013 #13

    bapowell

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    The cosmological constant, a special case of dark energy, indeed has constant energy density. More general forms of dark energy have densities that decrease with the expansion. So, it's not necessary that the fluid that drives accelerated expansion (if it even is a fluid) has a constant energy density.

    However, your objection confuses me. How is a fluid with constant energy density not logical? I see no violation of logic here. Of course, one could object on physical grounds, but it's not clear to me what those objections would be...

    Postulating causes for phenomena that lack understanding is how science works. There's much we have yet to figure out.
     
  15. Jul 24, 2013 #14
    Please believe me that I don't have objections just questions that are in the form of statements. I find statement exposes my thinking better than questions so others can help me in the way I think and what I know.

    =Is dark energy constant in energy density?=
    That was a question within a statement that questioned that idea. I think this is a miscommunication. Your response matches what I have been taught.

    Let me put my original inquiry differently-

    2001
    A cosmological surprise: the universe accelerates
    "The result from the distant supernovae that the expansion of the universe is not decelerating but in fact accelerates is certainly surprising"
    http://www.eso.org/~bleibund/papers/EPN/epn.html

    When acceleration was discovered it was totally unexpected and unpredicted. So what the postulated cause came to the rescue to account for the acceleration? Has the mechanism of the acceleration been explained? What new physics explained this totally new and unpredicted acceleration?


    It is my understanding from what cosmologist have said (ex. Alex Filippenko) is that dark energy does not interact with electromagnetism (thus matter) and physics can't explain the acceleration but it does exist and has a real effect on space. (Sounded like it is an "effect" without a "cause". Not a logical process to think about.)

    What explained the acceleration?
     
  16. Jul 24, 2013 #15

    bapowell

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    The cosmological constant was known to cause accelerated expansion when people first started looking for cosmological solutions to Einstein's Equation. Einstein himself included it as an explicit term in his equations to account for the (apparent at the time) static nature of the universe (the cosmological constant "pushed back" against the contraction of the matter-dominated, closed universe of Einstein's model). When the universe was later seen to be expanding, Einstein retracted the cosmological constant (referring to its inclusion as his "greatest blunder"). It is poignant that the cosmological constant might again be relevant, but for an accelerating, rather than static, spacetime.

    The microphysics leading to the existence of a cosmological constant is not currently known. Explanation in terms of quantum field theory face big problems, and other efforts to understand the accelerated expansion in terms of modifications to gravity have also faced difficulty.
     
  17. Jul 24, 2013 #16
    Ok lets drop dark energy and look at the cosmological constant itself. First off whether the universe contracts, expands of remains static is a relation between the critical density and the actual density. A static universe is one where the actual density exactly equals the critical density. However that is unstable. Einstein predicted either a contracting or expanding universe. He tried to make it static with his "greatest blunder" by adding the cosmological constant.

    [itex]\rho_{crit} = \frac{3c^2H^2}{8\pi G}[/itex]

    P=pressure
    c=speed of light
    G= gravitational constant.

    density is represented by the Greek letter Omega [itex]\Omega[/itex] so critical density is [itex]\Omega crit[/itex]
    total density is

    [itex]\Omega[/itex]total=[itex]\Omega[/itex]dark matter+[itex]\Omega[/itex]baryonic+[itex]\Omega[/itex]radiation+[itex]\Omega[/itex]relativistic radiation+[itex]{\Omega_ \Lambda}[/itex]

    [itex]\Lambda[/itex] or Lambda is the value of the cosmological constant often referred to as "dark energy" more accurately it is the vacuum pressure that attributes to expansion.
    Energy-density is the amount of energy stored per unit volume of space or region. Energy per unit volume has the same physical units as pressure, and in many circumstances is an exact synonym.

    [itex]\Omega=\frac{P_{total}}{P_{crit}}[/itex]
    or alternately
    [itex]\Omega=\frac{\Omega_{total}}{\Omega_{crit}}[/itex]

    This article describes the cosmological constant in a more complete manner, including alternate sources of the cosmological constant. I found it informative and interesting the section 522 has an example of how the cosmological constant is described as a fluid with a constant energy density and pressure .They also explore other possibilities in the article in particular, those described by related quantum mechanic, perturbation theory, supersymmetry, stress energy tensers etc.

    http://arxiv.org/pdf/1205.3365v1.pdf 15 May 2012
    "Everything You Always Wanted To Know About The Cosmological Constant Problem
    (But Were Afraid To Ask)"

    it is essentially a pedagogical review though does get a bit on the technical side.
     
  18. Jul 24, 2013 #17

    bapowell

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    This is where you need to be careful and likely where the OP is confused. Lambda is the cosmological constant -- it is a very special kind of dark energy, in that it can be described by a constant-density fluid in Einstein's Equations. Dark energy is not synonymous with the CC -- dark energy is more general and includes equations of state with time-varying density fluids.

    It is not clear that Lambda should be attributed to quantum vacuum energy -- nobody has gotten this interpretation to even remotely pass muster. Sure, the quantum vacuum should gravitate like a CC, but so far as our theories tell us, it's apparently not that simple.
     
  19. Jul 24, 2013 #18
    good point, its one where I will endeavor to be more clear on in the future. Lol personally I hate the term dark energy but that's just a side note. After looking at my earlier statements I can understand his confusion and for that I do apologize
     
    Last edited: Jul 24, 2013
  20. Jul 24, 2013 #19

    bapowell

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    Not a problem -- I too hate the term "dark energy".
     
  21. Jul 28, 2013 #20
    In a compleat or unified cosmological theory dark energy and dark matter are related by definition. If the big bang has validity they are related through the singularity from the start. If they are related at the start they are related now.

    There are logical suppositions about dark energy and dark matter but we know nothing, and that is why they are called dark. Dark matter could be an exotic form of matter or an exotic form of space. We associate gravity with matter yet gravity was the first force to be released before matter was even created through the strong and weak force. Dark energy could be a exotic form of gravity that expends space away from the center of gravity, call it ant-gravity.

    The point is that in a unified cosmology everything is related.
     
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