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In the 1920 American astronomer Edwin P. Hubble discovered that our universe isn’t static but was expanding. 

Ever since then, scientists have been trying to refine there measurement of size and rate of the universe’s expansion rate. However it is a hard thing to measurement.

Presently their three primary the methods used to determine its rate of expansion: parallax, standard candles and type Ia supernova.
Stellar parallax uses the apparent shift of position of any nearby star against the background of distant objects created by the different orbital positions of Earth.  Measurements are taken at six month intervals when the Earth arrives at exactly opposite sides of the Sun which give a baseline distance of about two astronomical units between observations. The parallax itself is considered to be half of this maximum, about equivalent to the observational shift that would occur due to the different positions of Earth and the Sun, a baseline of one astronomical unit (AU).

Once a star’s parallax is known, its distance from Earth can be computed trigonometrically. But the more distant an object is, the smaller its parallax. Even with 21st-century techniques in astrometry, the limits of accurate measurement make distances farther away than about 100 parsecs (roughly 326 light years) too approximate to be useful when obtained by this technique. Relatively close on a galactic scale, the applicability of stellar parallax leaves most astronomical distance measurements to be calculated by spectral red-shift or other methods.

The second method uses Standard candles called Cepheid variable stars, to measure distances throughout the Milky Way.  A variable star are ones who’s luminosity can be determined by how it brightness varies over time and comparing that to its apparent magnitude (how bright it looks to us) gives astronomers, by using the inverse square law they can infer the source’s distance simply by measuring the peak light output before comparing the number to its absolute magnitude. Using this mention they can accurately measure stellar distances up to around 1,000 MPC (parsecs),

Now, to measure extremely long distances, astronomers use type Ia supernova blasts as standard candles. They typically formed when two white-dwarf stars in a binary system collide, or one of them siphons enough material from its partner to temporarily reignite before ultimately exploding and becoming incinerated.  Astronomers have collected a lot of evidence that suggests the peak light output from one of these supernova blasts should always have an absolute magnitude of -19.6 which allows them to measure distances out to around 1000 Mpc, which is a significant fraction of the radius of the known Universe.

However measure distances using this technique is not as strait forward as it seems because their are many things in space that can absorb some of light’s energy thereby reducing a stars apparent brightness which would affect the distance measurements.

For example when the results of the Sloan Digital Sky Survey were analyzed it was found the colors of distant quasars that intergalactic space appears to be filled with a haze of tiny, smoke-like “dust” particles that dim the light from distant objects and subtly change their colors.

It goes on to say that this dust could also affect planned cosmological experiments that use supernovae to investigate the nature of “dark energy” which is causing the expansion of the universe to accelerate.  Scientists can use supernovae as a standard candle because it is an object that has some characteristic that allows us to determine its intrinsic luminosity.  Since the apparent luminosity or light which we receive has to do with the distance to the object, they can be used to figure out how far away an object is. For example if you know that a 60 watt light bulb gives off a certain amount of light or energy, and then measure the amount received from a one across the room from you, you could calculate the distance to it.

As mentioned earlier Astronomers can take advantage of standard candles to determine the distance to objects like galaxies. Using a type of supernova called a type Ia supernova, astronomers determined both the distance of the galaxy and the red shift of the galaxy. “Red shift” basically told them how much the Universe had expanded since the light left the supernova. The astronomers could then compare distance to expansion, and create a kind of ‘expansion history’ of the Universe.

However dust grains block blue light more effectively than red light. We find similar reddening of quasars from intergalactic dust, and this reddening extends up to ten times beyond the apparent edges of the galaxies themselves.  By analyzing the colors of large number or quasars located behind galaxies allows one to measure an effect that that dust has on the apparent brightness of interstellar objects. 

However there is another substance that might affect our understanding of the expansion rate of the universe.

For example we know that Dark Matter exists because of the gravitational effects it has on starts and galaxies. 

We also know that it is distributed throughout intergalactic space and that the force it projects like that of ordinary matter effects the energy of a photon because observations such as gravitational leasing tells us it does even though we may not know what it is made up of.

Therefore we should not assume that the mass associated with dark matter does interact with light casing it to loss energy or be red shifted similar to the way dust does.

However it would be much harder to detect because it would be more evenly distributed throughout space making the averaging technique used to determine the effect of dust on the apparent brightness of stars mentioned above even more difficult.

Yet if it does it could have a profound effect on our understanding of the universes expansion because even though its concentrations may very low relative to regions containing dust its distribution is more pervasive and therefore could have as large or even larger effect on the distance calculations based on the apparent brightness of quasars or type Ia supernova.

Later Jeff

Copyright Jeffrey O’Callaghan 2016

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