DOCSLIB.ORG

Galaxies at High Redshift Mauro Giavalisco

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Galaxies at High Redshift Mauro Giavalisco eaa.iop.org DOI: 10.1888/0333750888/1669 Galaxies at High Redshift Mauro Giavalisco From Encyclopedia of Astronomy & Astrophysics P. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888 Institute of Physics Publishing Bristol and Philadelphia Downloaded on Thu Mar 02 23:08:45 GMT 2006 [131.215.103.76] Terms and Conditions Galaxies at High Redshift E NCYCLOPEDIA OF A STRONOMY AND A STROPHYSICS Galaxies at High Redshift that is progressively higher for objects that are separated in space by larger distances. If the recession velocity between Galaxies at high REDSHIFT are very distant galaxies and, two objects is small compared with the speed of light, since light propagates through space at a finite speed of its value is directly proportional to the distance between approximately 300 000 km s−1, they appear to an observer them, namely v = H d on the Earth as they were in a very remote past, when r 0 the light departed them, carrying information on their H properties at that time. Observations of objects with very where the constant of proportionality 0 is called the high redshifts play a central role in cosmology because ‘HUBBLE CONSTANT’. For larger recession velocities this they provide insight into the epochs and the mechanisms relation is replaced by a more general one calculated from the theory of general relativity. In each cases, the value of GALAXY FORMATION, if one can reach redshifts that are high H enough to correspond to the cosmic epochs when galaxies of 0 provides the recession velocity of a pair of galaxies were forming their first populations of stars and began to separated by unitary distance, and hence sets the rate of shine light throughout space. the expansion. Since if the universe has been expanding One fundamental prediction of the theory of the at a higher rate it has taken a shorter time to go from big bang, which has found empirical confirmation in the size at the big bang (close to a mathematical point) the discovery of the cosmic background radiation, is to the current size, a higher value of the Hubble constant that early in its evolution the universe consisted only also means a younger universe. Recent measurements of of matter and radiation coupled in thermodynamical the Hubble constant place its value in the range between −1 −1 = . × 22 equilibrium and homogeneously distributed in space. No 60 and 70 km s Mpc (1 Mpc 3 1 10 m). This galaxies or stars or any other structure could exist in corresponds to an age of the universe between 10 and 15 = 9 such physical conditions, except for minuscule primordial Gyr (1 Gyr 10 yr). fluctuations of density, superimposed on the otherwise Aresult of the cosmic expansion and of the finite speed extraordinarily smooth distribution of matter and energy of light is that the light emitted from any given galaxy by quantum physical processes during the first instant is observed by a remote observer (i.e. one located on a of existence of the universe. Today, the universe is different galaxy, for example the Milky Way) as having highly inhomogeneous, with the matter organized in a a longer wavelength than it had at the emission. This hierarchy of structures such as stars, galaxies, clusters and is because the cosmic expansion has caused the space superclusters of galaxies. Understanding the mechanisms between the two galaxies to increase during the lapse of that led to the transition from the homogeneous early time between emission and detection, and while the light universe to the structured one observed at the present was traveling from one galaxy to the other. This causes epoch is of central importance to cosmology and a ‘stretching’ of the wavelength of the light, namely a fundamental physics. Yet, they remain poorly understood shift towards longer (therefore redder) wavelengths. The and still defeat empirical investigation. fractional change in the wavelength with respect to the Our current ideas attribute the formation of the one at emission as measured by the observer is called the cosmic structures to the action of gravity, which amplified ‘redshift’. It follows that, the higher the redshift, the longer the primordial density fluctuations until they collapsed the stretch in wavelength, and hence the more distant the under their own self-gravity and became bounded two galaxies. structures. According to this paradigm, the epoch The redshift provides a measure of the distance to a of galaxy formation and the properties of the nascent galaxy as well as of the lapse of time between the instant galaxies were decided by the properties and the relative of emission and that of observation. Thus, an observer abundances of the matter and the energy that permeated can see the galaxy and study its properties as it was at the the universe shortly after the big bang. Galaxies are the epoch of the emission. Galaxies located near ours have building blocks of the distribution of matter in space, and very small redshifts and are seen as they were a very an important step to decipher the history of the cosmic short time ago compared with the age of the universe. evolution is to identify the first galaxies that formed In practice, local galaxies are contemporaries of our own, in the universe and study their properties at the time and hence the present cosmic time corresponds to redshift of their formation. The search for ‘primeval galaxies’ zero. The value of the redshift of objects seen at the time has traditionally played a central role in observational of the big bang is infinity. Figure 1 shows the relationship cosmology for more than three decades and, even after between redshift, distance and the corresponding age of the spectacular accomplishments of the last 5 yr or so, is the universe (i.e. the lapse of time from the big bang still driving much of extragalactic research. to that redshift) for two different assumptions for the cosmological parameters and , namely the total The redshift and the expanding universe density of matter and the energy density due to the One of the most important scientific discoveries of this cosmological constant, respectively. In both cases the − − century is the expansion of the universe. This makes the Hubble constant has been assumed to be 65 km s 1 Mpc 1. galaxies and the other larger structures that populate the In practice, redshifts are measured from the spectra cosmic space recede away from each other with a velocity of the galaxies, namely diagrams that plot the intensity of Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK 1 Galaxies at High Redshift E NCYCLOPEDIA OF A STRONOMY AND A STROPHYSICS the sky are crowded by a myriad of relatively close but intrinsically faint galaxies, while intrinsically luminous galaxies at high redshift are comparatively very rare. This ‘contamination’ by interlopers is in fact so severe that, without some criterion to cull them from the faint, nearby ones, it would be totally impractical to search for high- redshift galaxies by randomly measuring the redshifts of samples of faint galaxies until the very distant ones are found. Because of this elusiveness, high-redshift galaxies came to be regarded as the ‘holy grail’ of cosmological research. Theoretical expectations of the mechanism of forma- tion of the so-called spheroids, namely ELLIPTICAL GALAXIES and bulges of SPIRAL GALAXIES, provided a criterion to iden- tify high-redshift galaxies. The stars that constitute the spheroids are among the oldest observed in the local uni- verse, and currently there is no appreciable ongoing activ- ity of STAR FORMATION in these systems. These facts indicate that the spheroids formed their stars during a very early Figure 1. The relationship between redshift, cosmic age and cosmic epoch in a relatively short period of time. Since distance for two different world models, both spatially flat (i.e. this epoch marked the first significant episodes of star with Euclidean geometry). The Hubble constant is assumed to H = −1 −1 formation during the cosmic evolution (note that about be 0 65 km s Mpc in both cases. In one model only ∼ / matter is present in space, namely = 1.0 (continuous curves); 1 2 of all the stars of the present-day universe are found in the other model the space is filled with both matter and the within spheroids), it has been traditionally identified with cosmological constant, with = 0.3 and = 0.7 (dashed the epoch of galaxy formation, and the nascent spheroids curves). The thin curves represent the cosmic age at the various themselves have been identified with the primeval galax- redshifts. The cosmic age at the present epoch (redshift zero), is ies. 10 and 14.5 billion years in the two models, respectively. At the The theoretical expectations predicted that the time of the big bang (infinite redshift) the cosmic age was zero. Within the accuracy of the plot, redshift z = 10 can already be entire stellar content of a spheroid formed during the identified as infinite. The thick curves represent the distance of a gravitational collapse of the proto-cloud of gas from which source as a function of its redshift, expressed as ×1000 Mpc. As a the structure originated. Simple physical arguments show comparison, the Andromeda galaxy is at about 0.8 Mpc from the that the duration of such a collapse is of the order of Milky Way and the Virgo cluster at about 18.5 Mpc. The most the time of free fall which, for a galaxy with the mass z ∼ . distant galaxies observed so far are located at a redshift 5 5.
Load more

Recommended publications
  • The Age of the Universe Transcript
    The Age of the Universe Transcript Date: Wednesday, 6 February 2013 - 1:00PM Location: Museum of London 6 February 2013 The Age of The Universe Professor Carolin Crawford Introduction The idea that the Universe might have an age is a relatively new concept, one that became recognised only during the past century. Even as it became understood that individual objects, such as stars, have finite lives surrounded by a birth and an end, the encompassing cosmos was always regarded as a static and eternal framework. The change in our thinking has underpinned cosmology, the science concerned with the structure and the evolution of the Universe as a whole. Before we turn to the whole cosmos then, let us start our story nearer to home, with the difficulty of solving what might appear a simpler problem, determining the age of the Earth. Age of the Earth The fact that our planet has evolved at all arose predominantly from the work of 19th century geologists, and in particular, the understanding of how sedimentary rocks had been set down as an accumulation of layers over extraordinarily long periods of time. The remains of creatures trapped in these layers as fossils clearly did not resemble any currently living, but there was disagreement about how long a time had passed since they had died. The cooling earth The first attempt to age the Earth based on physics rather than geology came from Lord Kelvin at the end of the 19th Century. Assuming that the whole planet would have started from a completely molten state, he then calculated how long it would take for the surface layers of Earth to cool to their present temperature.
    [Show full text]
  • Hubble's Law and the Expanding Universe
    COMMENTARY COMMENTARY Hubble’s Law and the expanding universe Neta A. Bahcall1 the expansion rate is constant in all direc- Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544 tions at any given time, this rate changes with time throughout the life of the uni- verse. When expressed as a function of cos- In one of the most famous classic papers presented the observational evidence for one H t in the annals of science, Edwin Hubble’s of science’s greatest discoveries—the expand- mic time, ( ), it is known as the Hubble 1929 PNAS article on the observed relation inguniverse.Hubbleshowedthatgalaxiesare Parameter. The expansion rate at the pres- between distance and recession velocity of receding away from us with a velocity that is ent time, Ho, is about 70 km/s/Mpc (where 1 Mpc = 106 parsec = 3.26 × 106 light-y). galaxies—the Hubble Law—unveiled the proportional to their distance from us: more The inverse of the Hubble Constant is the expanding universe and forever changed our distant galaxies recede faster than nearby gal- Hubble Time, tH = d/v = 1/H ; it reflects understanding of the cosmos. It inaugurated axies. Hubble’s classic graph of the observed o the time since a linear cosmic expansion has the field of observational cosmology that has velocity vs. distance for nearby galaxies is begun (extrapolating a linear Hubble Law uncovered an amazingly vast universe that presented in Fig. 1; this graph has become back to time t = 0); it is thus related to has been expanding and evolving for 14 bil- a scientific landmark that is regularly repro- the age of the Universe from the Big-Bang lion years and contains dark matter, dark duced in astronomy textbooks.
    [Show full text]
  • The Reionization of Cosmic Hydrogen by the First Galaxies Abstract 1
    David Goodstein’s Cosmology Book The Reionization of Cosmic Hydrogen by the First Galaxies Abraham Loeb Department of Astronomy, Harvard University, 60 Garden St., Cambridge MA, 02138 Abstract Cosmology is by now a mature experimental science. We are privileged to live at a time when the story of genesis (how the Universe started and developed) can be critically explored by direct observations. Looking deep into the Universe through powerful telescopes, we can see images of the Universe when it was younger because of the finite time it takes light to travel to us from distant sources. Existing data sets include an image of the Universe when it was 0.4 million years old (in the form of the cosmic microwave background), as well as images of individual galaxies when the Universe was older than a billion years. But there is a serious challenge: in between these two epochs was a period when the Universe was dark, stars had not yet formed, and the cosmic microwave background no longer traced the distribution of matter. And this is precisely the most interesting period, when the primordial soup evolved into the rich zoo of objects we now see. The observers are moving ahead along several fronts. The first involves the construction of large infrared telescopes on the ground and in space, that will provide us with new photos of the first galaxies. Current plans include ground-based telescopes which are 24-42 meter in diameter, and NASA’s successor to the Hubble Space Telescope, called the James Webb Space Telescope. In addition, several observational groups around the globe are constructing radio arrays that will be capable of mapping the three-dimensional distribution of cosmic hydrogen in the infant Universe.
    [Show full text]
  • The Anthropic Principle and Multiple Universe Hypotheses Oren Kreps
    The Anthropic Principle and Multiple Universe Hypotheses Oren Kreps Contents Abstract ........................................................................................................................................... 1 Introduction ..................................................................................................................................... 1 Section 1: The Fine-Tuning Argument and the Anthropic Principle .............................................. 3 The Improbability of a Life-Sustaining Universe ....................................................................... 3 Does God Explain Fine-Tuning? ................................................................................................ 4 The Anthropic Principle .............................................................................................................. 7 The Multiverse Premise ............................................................................................................ 10 Three Classes of Coincidence ................................................................................................... 13 Can The Existence of Sapient Life Justify the Multiverse? ...................................................... 16 How unlikely is fine-tuning? .................................................................................................... 17 Section 2: Multiverse Theories ..................................................................................................... 18 Many universes or all possible
    [Show full text]
  • Year 1 Cosmology Results from the Dark Energy Survey
    Year 1 Cosmology Results from the Dark Energy Survey Elisabeth Krause on behalf of the Dark Energy Survey collaboration TeVPA 2017, Columbus OH Our Simple Universe On large scales, the Universe can be modeled with remarkably few parameters age of the Universe geometry of space density of atoms density of matter amplitude of fluctuations scale dependence of fluctuations [of course, details often not quite as simple] Our Puzzling Universe Ordinary Matter “Dark Energy” accelerates the expansion 5% dominates the total energy density smoothly distributed 25% acceleration first measured by SN 1998 “Dark Matter” 70% Our Puzzling Universe Ordinary Matter “Dark Energy” accelerates the expansion 5% dominates the total energy density smoothly distributed 25% acceleration first measured by SN 1998 “Dark Matter” next frontier: understand cosmological constant Λ: w ≡P/ϱ=-1? 70% magnitude of Λ very surprising dynamic dark energy varying in time and space, w(a)? breakdown of GR? Theoretical Alternatives to Dark Energy Many new DE/modified gravity theories developed over last decades Most can be categorized based on how they break GR: The only local, second-order gravitational field equations that can be derived from a four-dimensional action that is constructed solely from the metric tensor, and admitting Bianchi identities, are GR + Λ. Lovelock’s theorem (1969) [subject to viability conditions] Theoretical Alternatives to Dark Energy Many new DE/modified gravity theories developed over last decades Most can be categorized based on how they break GR: The only local, second-order gravitational field equations that can be derived from a four-dimensional action that is constructed solely from the metric tensor, and admitting Bianchi identities, are GR + Λ.
    [Show full text]
  • Cosmic Microwave Background
    1 29. Cosmic Microwave Background 29. Cosmic Microwave Background Revised August 2019 by D. Scott (U. of British Columbia) and G.F. Smoot (HKUST; Paris U.; UC Berkeley; LBNL). 29.1 Introduction The energy content in electromagnetic radiation from beyond our Galaxy is dominated by the cosmic microwave background (CMB), discovered in 1965 [1]. The spectrum of the CMB is well described by a blackbody function with T = 2.7255 K. This spectral form is a main supporting pillar of the hot Big Bang model for the Universe. The lack of any observed deviations from a 7 blackbody spectrum constrains physical processes over cosmic history at redshifts z ∼< 10 (see earlier versions of this review). Currently the key CMB observable is the angular variation in temperature (or intensity) corre- lations, and to a growing extent polarization [2–4]. Since the first detection of these anisotropies by the Cosmic Background Explorer (COBE) satellite [5], there has been intense activity to map the sky at increasing levels of sensitivity and angular resolution by ground-based and balloon-borne measurements. These were joined in 2003 by the first results from NASA’s Wilkinson Microwave Anisotropy Probe (WMAP)[6], which were improved upon by analyses of data added every 2 years, culminating in the 9-year results [7]. In 2013 we had the first results [8] from the third generation CMB satellite, ESA’s Planck mission [9,10], which were enhanced by results from the 2015 Planck data release [11, 12], and then the final 2018 Planck data release [13, 14]. Additionally, CMB an- isotropies have been extended to smaller angular scales by ground-based experiments, particularly the Atacama Cosmology Telescope (ACT) [15] and the South Pole Telescope (SPT) [16].
    [Show full text]
  • Arxiv:2002.11464V1 [Physics.Gen-Ph] 13 Feb 2020 a Function of the Angles Θ and Φ in the Sky Is Shown in Fig
    Flat Space, Dark Energy, and the Cosmic Microwave Background Kevin Cahill Department of Physics and Astronomy University of New Mexico Albuquerque, New Mexico 87106 (Dated: February 27, 2020) This paper reviews some of the results of the Planck collaboration and shows how to compute the distance from the surface of last scattering, the distance from the farthest object that will ever be observed, and the maximum radius of a density fluctuation in the plasma of the CMB. It then explains how these distances together with well-known astronomical facts imply that space is flat or nearly flat and that dark energy is 69% of the energy of the universe. I. COSMIC MICROWAVE BACKGROUND RADIATION The cosmic microwave background (CMB) was predicted by Gamow in 1948 [1], estimated to be at a temperature of 5 K by Alpher and Herman in 1950 [2], and discovered by Penzias and Wilson in 1965 [3]. It has been observed in increasing detail by Roll and Wilkinson in 1966 [4], by the Cosmic Background Explorer (COBE) collaboration in 1989{1993 [5, 6], by the Wilkinson Microwave Anisotropy Probe (WMAP) collaboration in 2001{2013 [7, 8], and by the Planck collaboration in 2009{2019 [9{12]. The Planck collaboration measured CMB radiation at nine frequencies from 30 to 857 6 GHz by using a satellite at the Lagrange point L2 in the Earth's shadow some 1.5 10 km × farther from the Sun [11, 12]. Their plot of the temperature T (θ; φ) of the CMB radiation as arXiv:2002.11464v1 [physics.gen-ph] 13 Feb 2020 a function of the angles θ and φ in the sky is shown in Fig.
    [Show full text]
  • Lecture 14: the Age of the Universe
    The Age of the Universe One somewhat low-pro¯le but important test of cosmological models is that the age of the universe implied by the models agrees with the age estimated using various methods. Here we'll talk about the age we get from models, and in the next lecture will discuss several observational handles on the true age. For a change of pace, and to give us some practice in evaluating complicated equations, we will start with the equation for the lookback time provided by Hogg in astro-ph/9905116, and see whether the units, limits, etc. make sense. The lookback time from now (z = 0) back to redshift z means the time that would have elapsed on the wristwatch of an observer moving with the Hubble ow between those redshifts. Hogg's expression is: 1 z dz0 tL = 0 0 3 0 2 : (1) H0 Z0 (1 + z ) ­(1 + z ) + ­k(1 + z ) + ­¤ p Here H0 is the current value of the Hubble parameter, and ­, ­k, and ­¤ each indicate their current (z = 0) values. Does this expression behave as it should? See Figure 1 for a graphical representation. As always, we check units ¯rst. Recall that the Hubble parameter has units of inverse time, so the prefactor 1=H0 has units of time. The integral is dimensionless because the redshift is, so indeed the units are correct. Our second easy check is to note that tL must increase with increasing z. Since the integrand is always positive, this is indeed the case. Our third simple check is to note that smaller H0 should imply a larger time back to a given redshift, because H0 = a=a_ today.
    [Show full text]
  • The Hubble Constant
    Astro-2: History of the Universe Lecture 3; April 11 2013 Previously.. On Astro-2 • The universe is much larger than the Milky Way (Gpc vs kpc) • There are billion of galaxies, the Milky Way is average Joe galaxy • Most galaxies can be classified based on their appearance as – Elliptical – Lenticular – Spiral – Irregular • Spirals rotate and have young stars, gas and dust • Ellipticals do not rotate and have old stars, no gas nor dust Previously.. On Astro-2 • It is difficult but very important to figure out the size and distance of things in the universe • One way astronomers do that is by using “standard candles”. • Examples of standard candles are cepheids variable stars and supernovae Two important questions • Are there black holes in galaxies? • What is a black hole? Two important questions • How do we know it? • Exactly in the same way we know the mass of the sun, or the mass of galaxies for that matter! Two important questions • If the universe is homogeneous and isotropic how do stars know that there is a center of a galaxy to orbit around? • It is a matter of scale: • On scale much larger than a Mpc the universe is homogeneous • On scales much smaller than a Mpc the universe is NOT homogenous, there are galaxies, for example. Assignments. Due Friday 4/19 4PM 1. To TA: Universe 24.42 - 24.43 - 24.45 2. On your own: 24.17 thru 24.30 Today.. On Astro-2 1. How far are galaxies? 2. Measuring velocities and “redshifts”. 3. Hubble’s law.
    [Show full text]
  • 22. Big-Bang Cosmology
    1 22. Big-Bang Cosmology 22. Big-Bang Cosmology Revised August 2019 by K.A. Olive (Minnesota U.) and J.A. Peacock (Edinburgh U.). 22.1 Introduction to Standard Big-Bang Model The observed expansion of the Universe [1–3] is a natural (almost inevitable) result of any homogeneous and isotropic cosmological model based on general relativity. However, by itself, the Hubble expansion does not provide sufficient evidence for what we generally refer to as the Big-Bang model of cosmology. While general relativity is in principle capable of describing the cosmology of any given distribution of matter, it is extremely fortunate that our Universe appears to be homogeneous and isotropic on large scales. Together, homogeneity and isotropy allow us to extend the Copernican Principle to the Cosmological Principle, stating that all spatial positions in the Universe are essentially equivalent. The formulation of the Big-Bang model began in the 1940s with the work of George Gamow and his collaborators, Ralph Alpher and Robert Herman. In order to account for the possibility that the abundances of the elements had a cosmological origin, they proposed that the early Universe was once very hot and dense (enough so as to allow for the nucleosynthetic processing of hydrogen), and has subsequently expanded and cooled to its present state [4,5]. In 1948, Alpher and Herman predicted that a direct consequence of this model is the presence of a relic background radiation with a temperature of order a few K [6,7]. Of course this radiation was observed 16 years later as the Cosmic Microwave Background (CMB) [8].
    [Show full text]
  • The Problem of the Age of the Universe and the Earliest Galaxies
    The Problem of the Age of the Universe and the Earliest Galaxies Roger Ellman Abstract A number of years ago the estimates of astronomers and astrophysicists were that the earliest galaxies took about 2½ - 3 billion years to form, that is, that they did not appear until 2.5-3.0 billion years after the Big Bang. Those estimates were based on analysis of the processes involved in star formation and in the aggregation and “clumping” of matter in the early universe. Since then improved equipment and techniques [e.g. Keck and Hubble telescopes and gravitational lensing] have resulted in reports of observation of early galaxies having stars that formed as early as 300 million years after the Big Bang. Such new data has led to the abandonment of the several billion years estimates of the time required for star and galaxy formation; however, an alternative response to the data would be to re-examine the Hubble theory from which the age of the universe and the distance to high redshift objects is determined. Roger Ellman, The-Origin Foundation, Inc. 320 Gemma Circle, Santa Rosa, CA 95404, USA [email protected] http://www.The-Origin.org 1 The Problem of the Age of the Universe and the Earliest Galaxies Roger Ellman Background of the Problem A number of years ago the estimates of astronomers and astrophysicists were that the earliest galaxies took about 2½ - 3 billion years to form, that is, that they did not appear until 2.5-3.0 billion years after the Big Bang. Those estimates were based on analysis of the processes involved in star formation and in the aggregation and “clumping” of matter in the early universe.
    [Show full text]
  • Evidence for the Big Bang
    fact sheet Evidence for the Big Bang What is the Big Bang theory? The Big Bang theory is an explanation of the early development of the Universe. According to this theory the Universe expanded from an extremely small, extremely hot, and extremely dense state. Since then it has expanded and become less dense and cooler. The Big Bang is the best model used by astronomers to explain the creation of matter, space and time 13.7 billion years ago. What evidence is there to support the Big Bang theory? Two major scientific discoveries provide strong support for the Big Bang theory: • Hubble’s discovery in the 1920s of a relationship between a galaxy’s distance from Earth and its speed; and • the discovery in the 1960s of cosmic microwave background radiation. When scientists talk about the expanding Universe, they mean that it has been increasing in size ever since the Big Bang. But what exactly is getting bigger? Galaxies, stars, planets and the things on them like buildings, cars and people aren’t getting bigger. Their size is controlled by the strength of the fundamental forces that hold atoms and sub-atomic particles together, and as far as we know that hasn’t changed. Instead it’s the space between galaxies that’s increasing – they’re getting further apart as space itself expands. How do we know the Universe is expanding? Early in the 20th century the Universe was thought to be static: always the same size, neither expanding nor contracting. But in 1924 astronomer Edwin Hubble used a technique pioneered by Henrietta Leavitt to measure distances to remote objects in the sky.
    [Show full text]