| Abstract from DBPedia | Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving away from Earth. The velocity of the galaxies has been determined by their redshift, a shift of the light they emit toward the red end of the visible spectrum. Hubble's law is considered the first observational basis for the expansion of the universe, and today it serves as one of the pieces of evidence most often cited in support of the Big Bang model.The motion of astronomical objects due solely to this expansion is known as the Hubble flow. It is described by the equation v = H0D, with H0 the constant of proportionality—the Hubble constant—between the "proper distance" D to a galaxy, which can change over time, unlike the comoving distance, and its speed of separation v, i.e. the derivative of proper distance with respect to the cosmological time coordinate. (See Comoving and proper distances § Uses of the proper distance for some discussion of the subtleties of this definition of "velocity".) The Hubble constant is most frequently quoted in (km/s)/Mpc, thus giving the speed in km/s of a galaxy 1 megaparsec (3.09×1019 km) away, and its value is about 70 (km/s)/Mpc. However, the SI unit of H0 is simply s−1, and the SI unit for the reciprocal of H0 is simply the second. The reciprocal of H0 is known as the . The Hubble constant can also be interpreted as the relative rate of expansion. In this form H0 = 7%/Gyr, meaning that at the current rate of expansion it takes a billion years for an unbound structure to grow by 7%. Although widely attributed to Edwin Hubble, the notion of the universe expanding at a calculable rate was first derived from general relativity equations in 1922 by Alexander Friedmann. Friedmann published a set of equations, now known as the Friedmann equations, showing that the universe might be expanding, and presenting the expansion speed if that were the case. Then Georges Lemaître, in a 1927 article, independently derived that the universe might be expanding, observed the proportionality between recessional velocity of, and distance to, distant bodies, and suggested an estimated value for the proportionality constant; this constant, when Edwin Hubble confirmed the existence of cosmic expansion and determined a more accurate value for it two years later, came to be known by his name as the Hubble constant. Hubble inferred the recession velocity of the objects from their redshifts, many of which were earlier measured and related to velocity by Vesto Slipher in 1917. Though the Hubble constant H0 is roughly constant in the velocity-distance space at any given moment in time, the Hubble parameter H, of which the Hubble constant is the current value, varies with time, so the term constant is sometimes thought of as somewhat of a misnomer.ハッブル＝ルメートルの法則（旧称「ハッブルの法則」）とは、現代宇宙論において観測される以下の法則のことを指す。 1.
* 10メガパーセク以上離れた位置に観測される天体には、地球との相対速度と解釈される赤方偏移が見られる。 2.
* このドップラー効果から計算される、様々な銀河の地球からの後退速度は、数百メガパーセク程度の銀河までは地球からの距離にほぼ比例する。 を天体が我々から遠ざかる速さ（後退速度）、 を我々からその天体までの距離とすると、 となる。ここで比例定数 はハッブル定数 (Hubble constant) と呼ばれ、現在の宇宙の膨張速度を決める。ハッブル定数は時間の逆数の次元 T−1 をもち、通常はキロメートル毎秒毎メガパーセク（記号: km/s/Mpc）が単位として用いられる。この発見は、宇宙は膨張しているものであるとする説を強力に支持するものとなった。 1929年にエドウィン・ハッブルが論文で発表したため、この法則は「ハッブルの法則」の名称で広く世に知られたが、1922年に現在「フリードマン方程式」として知られている一連の方程式を発表したアレクサンドル・フリードマンによって一般相対性理論の式から導き出されたのが最初である。1927年にはジョルジュ・ルメートルも宇宙の膨張を提案し、膨張率の推定値を提示していたが、フランス語のマイナーな雑誌に掲載されたため、その時点ではまだ注目されていなかった。なお、ルメートルはスライファーとハッブルの観測データを用いている。2018年8月に開催された第30回国際天文学連合総会で、ルメートルの功績を顕彰するため、ハッブルの法則を「ハッブル＝ルメートルの法則」 (Hubble-Lemaître law) と呼ぶことを推奨する決議案が提出された。2018年10月26日に締め切られた投票の結果、この決議案は約78%の賛成多数を以て採択された。 (Source: http://dbpedia.org/resource/Hubble's_law) |
