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Speed of light , age, etc.

  1. Apr 27, 2013 #1
    hello there , now excuse me to those to whom this looks like the questions you have hard a thousand times before.
    Now I understand how we calculate the speed of light in vacuum and no doubt we can do that under laboratory conditions etc.
    Light bending in gravitational fields is also understood.

    Now I understand that we have other ways of telling the approximate age of the universe , but here I want to ask about the one which involves distant stars and objects and light traveling from them.
    Now it is clear that as both the observer (earth) and the distant star move ahead in time simultaneously , when the observer from earth suddenly wants to take a look at the distant star he sees the star as it was when the light, that now has came to the observers "eye", left the distant star.So from that we take to calculate how long the light has traveled from the distant star hence we get the age.Now when we look the furthest possible we can see , we say that this light that we saw has traveled that and that years and "voula" we get the age of the universe.

    Now after all that I just said I have a hard time understanding how can we calculate and say the time it took for the light to come to us if we don't know the distance it has traveled ? I'm not a mathematics genius but I can tell that just by knowing the speed of a car you can't tell how long it has been traveling at that or other speeds.

    The difficulty here for me is that we know the light speed in vacuum and we can measure that , we also can tell the frequency change due to red or blue shift (gravity) but how can we tell how far hence old is the light that has come to us if the photons that hit our "eye" are just that photons emitted from a source a "while" ago but they don't tell about themselves nothing more than just the speed and frequency of them and speaking in big terms the "picture" they show us.

    maybe I'm missing out a stupid little but very important thing here so help me out.?
  2. jcsd
  3. Apr 27, 2013 #2


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    There is no (direct) way to calculate "how long did light travel?". You can estimate the distance* based on the brightness (real and apparent) of the object. You have to combine this with redshift measurements to get the age of the universe. It is not necessary to use the oldest known objects for those measurements. The oldest known objects (I don't count the cosmic microwave background as "object" here) are younger than the age of the universe, and this difference can be measured.

    * or some value related to the distance, as the distance is increasing all the time
  4. Apr 27, 2013 #3
    Aren't the exploding stars (supernova) serving us as a cosmic 'candles', for which we know standard universal brightness, and based on the strength/weakness of brightness we receive we can deduce distance to them?

    And on the cosmic scale there are many supernova every night...
  5. Apr 28, 2013 #4
    Well do those "candles" do us any good or the light itself , because there is another factor to be taken into account - the expansion of the universe , the whole medium in which light itself travels.
    Maybe somebody has some commentary on this one.
  6. Apr 28, 2013 #5
    Distance measures is a tricky subject. Thankfully I and other PF members took the time to write up an article covering it.
    Yes its long but very informative.

    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
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