Observable Universe

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Artist's representation of the observable universe in logarithmic scale and centering on the solar system. The inner and outer planets of the solar system , the Kuiper Belt , the Oort Cloud , Alpha Centauri , the Perseus Arm , the Milky Way , the Andromeda Nebula , neighboring galaxies , filaments and voids , the cosmic background radiation and the state of plasma shortly after the Big Bang are shown .

In the standard model of cosmology, the observable universe is that part of the universe that is in principle accessible to our observation from Earth. Assuming that the universe is isotropic , the universe observable from earth has the shape of a sphere with the observer on earth in the center. This is independent of the shape of the universe as a whole. Every place in the universe has its own observable universe, which may or may not overlap with ours. There are different approaches to determine the radius of this sphere.

Observation horizon

The observation horizon or particle horizon limits the part of the universe from which information has reached us since the Big Bang .

However, the distance to the observation horizon is not given by the age of the universe multiplied by the speed of light. So it's not 13.8 billion light years . According to the Big Bang Standard Model, it is estimated at 46.6 billion light years. It must be borne in mind that the universe further expanded has, while the light from the observation horizon moved to Earth, d. In other words, distances already covered have subsequently become longer. The most distant objects whose light we can perceive today were at the time when they emitted this light, at a distance of just 40 million light years from Earth - hardly closer than the event horizon of that time. Today we are separated from these objects by the aforementioned 46.6 billion light years. But since they have long since passed the event horizon, there is no way of ever knowing anything about what is going on at this distance. The ratio of these distances is the factor of the expansion of the universe over this period and at the same time the redshift .

Often the equivalent, reverse view is also used for the definition: The particle horizon is then the spherical surface up to which light-fast radiation would have penetrated if it had been emitted from our point of view immediately after the Big Bang and had been able to spread unhindered.

Relationship with the scale factor

The distance to today's particle horizon results from the integral over the reciprocal of the scale function:

where is the speed of light and the time that is set to zero during the Big Bang . is equal to today's age. The size is the scale factor . This is a dimensionless quantity whose course over time indicates the expansion of the universe. is the value of the scale factor at . So it applies .

If the integral diverges for a given one, the associated universe has no particle horizon.

Cosmic background radiation as a border

Simplified representation of the observable universe:
time axis upwards, spatial dimensions horizontally outwards;
Theoretical observation horizon as external and cosmic background radiation or actual observation horizon as an inner cone,
between the two - shown in light gray - the opaque plasma.
The uneven expansion of the universe over time is not shown.
Representation of the observable universe and its horizons, taking into account the uneven expansion of the universe over time. The gray dashed lines show the Hubble flow.

In practice, the observation horizon is slightly closer to electromagnetic radiation at most wavelengths because the early universe was opaque to light. The most distant information, and thus the information about the most distant areas, that can be obtained via electromagnetic waves, comes from around 380,000 years after the Big Bang, when the universe became transparent. This radiation, known as cosmic background radiation, comes from the edge of the universe that can be observed today.

Redshift

The radiation that we observe from an object is more redshifted the closer it is to the observation horizon. For objects directly on the observation horizon, the redshift is infinite. However, it is wrong to assume that objects on the observation horizon are moving away from us at the speed of light today, as one might mean when interpreting the cosmological redshift as a Doppler effect . Objects on the observation horizon seem to move away from us at more than 3 times the speed of light. This does not contradict the theory of relativity , because the expansion of the universe is not a movement in space, but an expansion of space itself. Today, as I said, it is no longer possible to transfer information from an object on the observation horizon to us (or vice versa).

Hubble radius

The Hubble radius describes the distance where the speed of a distant galaxy due to the expansion of the universe corresponds to the speed of light c . The corresponding sphere is accordingly also called the Hubble sphere. The speed due to the expansion of the universe is called the escape or recession speed.

The Hubble radius is inversely proportional to the Hubble parameter and is calculated according to:

Moving objects within the Hubble sphere move away from the earth with a speed less than c, objects outside with faster than light speed. If there is a peculiar speed in addition to the escape speed , this can be added. For example, an outgoing photon at the Hubble radius would move away from us with 2 c and an incoming photon with 0 c .

The current Hubble radius is about 14.2 billion light years, which corresponds to a redshift of z  = 1.46. According to the standard model of cosmology, the Hubble radius starts with the value zero at the Big Bang, then rises sharply to today's value and will stagnate in a few billion years at a value slightly above today's value. The Hubble parameter has accordingly fallen sharply since the Big Bang and is still falling today.

Event horizon

The event horizon indicates how far an object may be away from us today, so that its light can in principle just reach us in a theoretical limit value in the infinite future. At first it seems that this distance should be congruent with the Hubble radius, since light rays can never reach us if they are emitted in an area that moves away from us at faster than light speeds. In fact, the event horizon is a little further away. In the Standard Model, the event horizon is 16.2 billion light years. Due to the dynamics of the expansion of the universe, the Hubble radius grows over time. Therefore, in the future, some objects will be observable that are too far away today.

However, events and objects behind the cosmological event horizon are not causally related to us. No information from them can reach us. Even in the distant future, you will not be immersed in the growing Hubble radius, but will have reached a greater distance beforehand. Objects that are already visible will also escape the visible Hubble radius over time due to the expansion of space.

Web links

Individual evidence

  1. ^ J. Richard Gott III, Mario Jurić, David Schlegel u. a .: A Map of the Universe. On: astro.princeton.edu. 2005 (English; PDF; 3.7 MB).
  2. ^ Frequently Asked Questions in Cosmology. (English). On: Astro.ucla.edu. Retrieved April 20, 2013.
  3. Torsten Fließbach: General Theory of Relativity. 4th edition, ISBN 3-8274-1356-7 , p. 321.
  4. ^ A b Tamara M. Davis, Charles H. Lineweaver: Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the universe . In: Publications of the Astronomical Society of Australia . 21, No. 1, 2004, p. 97. arxiv : astro-ph / 0310808 . bibcode : 2004PASA ... 21 ... 97D . doi : 10.1071 / AS03040 .
  5. ^ Tamara Davis, Charles Lineweaver: Expanding Confusion. P. 3, arxiv : astro-ph / 0310808
  6. ^ Tamara Davis, Charles Lineweaver: Expanding Confusion. P. 18, arxiv : astro-ph / 0310808
  7. ^ Charles Lineweaver: Misconceptions about the Big Bang . Scientific American. 2005. Retrieved November 6, 2008.