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  2. Sjur Refsdal - Wikipedia

    en.wikipedia.org/wiki/Sjur_Refsdal

    In 1964 and 1966 he published a series of articles on the effects and possible applications of gravitational lenses. [2] He is particularly known for the "Refsdal Method", which describes how one may estimate the expansion rate of the Universe ( Hubble constant ) using the measured time-delay and lens properties of a gravitationally lensed ...

  3. Gravitational lensing formalism - Wikipedia

    en.wikipedia.org/wiki/Gravitational_lensing...

    In weak lensing by large-scale structure, the thin-lens approximation may break down, and low-density extended structures may not be well approximated by multiple thin-lens planes. In this case, the deflection can be derived by instead assuming that the gravitational potential is slowly varying everywhere (for this reason, this approximation is ...

  4. Gravitational lens - Wikipedia

    en.wikipedia.org/wiki/Gravitational_lens

    In 1963 Yu. G. Klimov, S. Liebes, and Sjur Refsdal recognized independently that quasars are an ideal light source for the gravitational lens effect. [23] It was not until 1979 that the first gravitational lens would be discovered. It became known as the "Twin QSO" since it initially looked like two identical quasistellar objects.

  5. Boussinesq approximation (buoyancy) - Wikipedia

    en.wikipedia.org/wiki/Boussinesq_approximation...

    The Boussinesq approximation is inaccurate when the dimensionless density difference ⁠ Δρ / ρ ⁠ is approximately 1, i.e. Δρ ≈ ρ. For example, consider an open window in a warm room. The warm air inside is less dense than the cold air outside, which flows into the room and down towards the floor.

  6. External ballistics - Wikipedia

    en.wikipedia.org/wiki/External_ballistics

    Air pressure, temperature, and humidity variations make up the ambient air density. Humidity has a counter intuitive impact. Humidity has a counter intuitive impact. Since water vapor has a density of 0.8 grams per litre, while dry air averages about 1.225 grams per litre, higher humidity actually decreases the air density, and therefore ...

  7. Introduction to general relativity - Wikipedia

    en.wikipedia.org/wiki/Introduction_to_general...

    In particular, gravitational lensing provides one way to measure the distribution of dark matter, which does not give off light and can be observed only by its gravitational effects. One particularly interesting application are large-scale observations, where the lensing masses are spread out over a significant fraction of the observable ...

  8. Tolman–Oppenheimer–Volkoff equation - Wikipedia

    en.wikipedia.org/wiki/Tolman–Oppenheimer...

    Let us assume a static, spherically symmetric perfect fluid. The metric components are similar to those for the Schwarzschild metric: [2] = = ⁡ By the perfect fluid assumption, the stress-energy tensor is diagonal (in the central spherical coordinate system), with eigenvalues of energy density and pressure:

  9. Weak gravitational lensing - Wikipedia

    en.wikipedia.org/wiki/Weak_gravitational_lensing

    The effects of foreground galaxy cluster mass on background galaxy shapes. The upper left panel shows (projected onto the plane of the sky) the shapes of cluster members (in yellow) and background galaxies (in white), ignoring the effects of weak lensing. The lower right panel shows this same scenario, but includes the effects of lensing.