Overview

age of the Universe

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cosmological tests

Overview page. Subjects: Astronomy and Astrophysics.

The purpose of cosmological tests is to derive constraints on the cosmological parameters from observations such as counts and angular sizes of distant objects such as galaxies and quasars....

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Einstein–de Sitter universe

Overview page. Subjects: Astronomy and Astrophysics.

A type of universe in which the mean density of matter is precisely matched to the critical density. Such a model will not actually collapse, but will expand for ever with a continually...

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galaxy evolution

Overview page. Subjects: Astronomy and Astrophysics.

Any change in the properties of individual galaxies, or of populations of galaxies, with time. The properties of galaxies in the Universe today provide evidence that galaxy evolution does...

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globular cluster

Overview page. Subjects: Astronomy and Astrophysics.

A roughly spherical group of old stars in the halo of a galaxy. Globular clusters contain from tens of thousands to millions of stars, and have diameters of 100–300  l.y. At the centre of...

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horizon distance

Overview page. Subjects: Astronomy and Astrophysics.

The horizon distance is the maximum distance from which we can receive a signal from a distant object or phenomenon in the Universe. Because the Universe has a finite age ...

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Hubble constant

Overview page. Subjects: Astronomy and Astrophysics.

(H 0)

The figure that relates the speed of an object's recession in the expanding Universe to its distance in the Hubble law. It represents the current rate of...

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Hubble Space Telescope

Overview page. Subjects: Astronomy and Astrophysics.

A 2.4-m reflector built by NASA and ESA, launched in 1990 April. It orbits at an altitude of about 600 km. An initial servicing mission by astronauts in 1993 December was necessary to...

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Hubble time

Overview page. Subjects: Astronomy and Astrophysics.

The time required for the Universe to expand to its present size, assuming that the Hubble constant has remained unchanged since the Big Bang. It is defined as the reciprocal of the Hubble...

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light cone

Overview page. Subjects: Astronomy and Astrophysics.

A means of illustrating the past and future of an event in spacetime. Spacetime diagrams in three dimensions are conventionally depicted with time as the vertical axis, and the other two...

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lookback time

Overview page. Subjects: Astronomy and Astrophysics.

The time taken for light from a distant object to reach the Earth; sometimes also known as light travel time. Light from nearby galaxies takes several million years to reach us, but the...

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