Expansion of the Universe

• Type of physical science: Astronomy; Astrophysics
• Field of study: Galaxies

The galaxies of the universe are traveling away from one another, with the most distant galaxies receding at the greatest velocities. Understanding the expansion of the universe will lead to an understanding of its beginning and of its ultimate fate.

History

The universe is composed of stars, including Earth's sun, clumped together by the billions in groups called galaxies. Billions of galaxies are scattered like islands throughout the sea of space. In 1929, it was discovered that the galaxies were not fixed in place but were speeding away from one another; the more distant the galaxy, the greater its speed. This is commonly visualized by representing galaxies as raisins in a loaf of raisin bread; each raisin (galaxy) moves away from all the other raisins as the bread rises. The observed galactic motion says that the universe is expanding, and also implies that the universe had a definite beginning. This one observation revolutionized the ideas on the nature of the universe, of its beginning, and of its ultimate fate.

Looking up at the night sky, the expansion cannot be seen. Although the Moon, stars, and planets rotate overhead with the hours and the seasons, their patterns have remained unchanged throughout human history. It has taken a combination of theory and observation to lead from the idea of a static universe, as explained by the Greeks, to the dynamic expanding universe of the twentieth century.

In 1610, Galileo Galilei turned the newly invented telescope toward the sky and discovered not only that millions of stars formed the Milky Way but also that some objects were not starlike. These objects were called nebulas, from the Latin for "cloud." These spiral and spiderlike shapes were thought to be clouds of dust and gas. In the mid-1700s, philosopher Immanuel Kant suggested that the spiral nebulas were composed of individual stars, like the Milky Way, and were actually "island universes" scattered about the vast void of space. Although Kant's idea, partly based on a work by English philosopher Thomas Wright, was reasoned to be intuition, not observation, it inspired later astronomers.

In the 1800s, advancements in observational astronomy provided the techniques needed to uncover the nature of the nebulas. Physicist William Hyde Wollaston discovered the presence of dark lines in the spectrum of colors that make up sunlight. These lines represent the absorption of certain frequencies (colors) characteristic of elements in the sun's outer layers.

Using this information, astronomer Sir William Huggins used the spectroscope (an instrument to observe spectra) to study the spectra of nebulas. He discovered two kinds of nebulas: those with the spectra of heated gas and those with the spectra of stars. Throughout the nineteenth century, astronomers sought to improve the resolution of their telescopes as they cataloged stars and nebulas and collected their spectra.

The main stumbling block of the time was the size of the Milky Way. Astronomers simply did not know the distance to most objects they saw. Distances to a few hundred stars were known, worked out by the parallax method. "Parallax" is the apparent motion of an object, compared to distant ones, because of the motion of the observer. This apparent motion can be seen by looking at one's finger on an outstretched arm first with one eye and then with the other. The finger appears to move, compared to distant buildings or trees. Using the motion of Earth around the sun, the distance to nearby stars can be found. Yet, this works only for stars a few tens of light-years away. (A light-year is the distance light travels in one year, which is equal to 9,460 billion kilometers or 5,880 billion miles)

Evidence

In the 1890s, Henrietta Swan Leavitt, at the Harvard College Observatory, discovered a relation between brightness (luminosity) and period for Cepheid variable stars. Variable stars change their brightness with time. The time for one cycle of change is the period. For Cepheid variables, the brighter the star is, the longer its period. The observed brightness of any star decreases regularly with distance. Measuring the period of a Cepheid determines its brightness. Therefore, by comparing it to a star of known brightness and distance, the distance to the Cepheid can be found.

In the early twentieth century, Harlow Shapley, at Mount Wilson Observatory, used the method to show the Milky Way as a disk-shaped collection of stars and gas surrounded by a sphere of globular star clusters. Beginning in 1919, Edwin Powell Hubble began to study spiral nebulas, sorting them into distinct categories. In 1923, using the newly completed 254-centimeter (100-inch) reflector telescope, Hubble found individual stars—Cepheid variables—in the Andromeda nebula.

Over the next two years, Hubble discovered fifty more Cepheids in nebula NGC 6822. Using Shapley's distance method, Hubble found these nebulas to be several hundred thousand light-years away. He also photographed nebula M33 with new, sensitive film, resolving it into distinct stars. Thus, he proved that nebulas were independent galaxies, apart from the Milky Way.

As Hubble looked for Cepheid variables, he also collected the nebular spectra. Although the spectra resembled that of the stars, they also differed. The dark absorption lines for the elements were found in the same order as in stellar spectra but shifted from the expected frequencies (or colors). The spectra of light from the galaxies is subject to the Doppler effect, or shift, if the galaxy is moving toward or away from an observer. Hubble found a few galactic spectra shifted toward the shorter-wavelength, or blue, end of the spectrum, meaning they were traveling toward the Milky Way. However, Doppler shifts for the vast majority of galaxies were toward the red wavelengths, meaning the galaxies were traveling away from the Milky Way.

In 1929, Hubble and fellow astronomer Vesto Melvin Slipher had forty-six redshift measurements for eighteen galaxies and the Virgo star cluster. By plotting the data, Hubble concluded that redshift (and velocity) were proportional to distance. The relation is now known as Hubble's law or the Hubble–Lemaître law. Thus, an object's velocity depends on its distance; the greater the velocity, the farther away the object is. Further confirmation was provided by Milton Humason at Mount Wilson. By 1935, Humason had added 150 redshifts to the list, with velocities in excess of one-eighth the speed of light. With the completion of the 508-centimeter (200-inch) telescope, Humason found, by the late 1950s, redshifted galaxies traveling at one-third the speed of light at a distance of several billion light-years. Hubble and Humason found that all but the nearest galaxies were rushing away from the Milky Way at tremendous speeds. They discovered that the universe was expanding, with the distance between galaxies steadily growing larger.

An expanding universe had been predicted independently before Hubble's discovery by Soviet mathematician Aleksandr Aleksandrovich Friedmann and Dutch astronomer Willem de Sitter, as a solution to Albert Einstein's general theory of relativity. Nevertheless, the idea of such a universe was difficult to accept. Even Einstein rejected his own solution for an expanding universe because of the lack of observational data. The observations of Hubble and Humason, however, forced astronomers to accept a new view of the universe.

Two basic models evolved to explain the observed expansion. In the steady-state theory, as the galaxies expand, new matter is created to fill the space between the galaxies. Such a universe has neither beginning nor end; it is expanding but unchanging. The Big Bang theory proposed a universe expanding outward from an initial explosion at some time in the past. Both models had supporters throughout the 1930s and 1940s.

Nevertheless, as astronomers tested the models against more and better observations, evidence mounted to show that the universe was expanding from an early high-density state. The time since this explosion can be found from Hubble's law. If a galaxy's velocity is proportional to its distance, then at some time in the past, all the galaxies were close together. The slope of the redshift (velocity) versus distance relation is called the Hubble constant. Taking the reciprocal of the Hubble's constant, one can calculate how long the galaxies have been moving apart. Because of uncertainties in the data and revision of distance estimates, most astronomers suggest that the universe began with an initial explosion between 10 and 20 billion years ago.

Impact

The concept of the expanding universe promoted technological development of new techniques to study distant stars and galaxies. Astronomers look into space not only in the visible part of the spectrum but also in the infrared, ultraviolet, X-ray, and radio wavelengths. Detailed study of the expansion of the universe has prompted the need for larger telescopes (such as the Hubble Space Telescope) with better resolution in visible and nonvisible wavelengths. Seeing the fainter, more distant galaxies gives information farther back in time. This has a two-fold use: it provides scientists with information on the early history of the universe and allows them to predict the universe's ultimate fate more accurately.

The discovery of the expansion of the universe forced a revolutionary change in scientists' ideas about the universe. Previously, the question of the origin of the universe was a subject for metaphysics or theology. With Hubble and Humason's discovery, questions regarding the beginning of the universe moved into the realm of science. The increased understanding of stars and galaxies as one looks farther away in distance and time has forced scientists to look more deeply into the two basic theories of the universe: the general theory of relativity and quantum mechanics. A major goal of many researchers is to combine these theories into one unified explanation of the universe.

Astronomers have realized that Hubble's law is not strictly linear. The expansion must slow with time because of the gravitational attraction of the mass of the universe. Scientists building on Friedmann's work have discovered a family of expanding models for the universe. The difference in the models depends, in part, on the total mass of the universe. If the mass is great enough, the universe will slow to a stop and eventually collapse; otherwise, the universe will continue to expand forever. Astronomers continue to attempt to determine the rate of expansion in order to learn the future of the universe.

An important development in the field came in the late 1990s, when multiple research teams presented evidence that not only supported the expanding universe model, but also indicated the rate of expansion was accelerating rather than slowing. This helped foster speculation about potential dark energy or other mysterious forces that could account for such accelerating expansion. Many experts believe the universe expanded extremely rapidly in a short time after the Big Bang, then expanded more slowly until about 4 billion years ago, when the expansion began to accelerate.

The questions involved only heightened as new ways of measuring the Hubble constant, including measurements based on the cosmic microwave background (CMB), produced significant discrepancies in the estimated expansion rate of the universe. For example, CMB data from the European Space Agency's Planck satellite led to a calculated rate of 67.4 kilometers (41.9 miles) per second per megaparsec, but measurements released in 2019 based on Cepheid variables suggested 74 km/sec/Mpc (46 miles/sec/Mpc). Later in 2019, a research team attempting to resolve the disagreement with a unique measurement method based on red giant stars came up with a third estimate (69.8 km/sec/Mpc or 43.4 miles/sec/Mpc) that did not strongly favor either previous estimate.

Some scientists suggested that the disparity, often called Hubble tension, required a complete rethinking of accepted cosmological models. Nobel laureate Adam Riess led the largest research study of Hubble tension ever conducted using the James Webb Space Telescope in December 2024. The findings confirmed Hubble's previous measurements from Earth to distant galaxies, indicating an expansion rate around 8 percent faster than the predictions made by theoretical models. This confirmation suggested that a gap in the modern scientific understanding of dark matter, dark energy, or gravity may be the source of Hubble tension rather than observational errors, as previously thought.

Further plans to investigate the expansion of the universe continued the following year, with the National Aeronautics and Space Administration’s Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer (SPHEREx) mission to survey optical and near-infrared light in the Milky Way. Scientists aimed to use data from the SPHEREx telescope’s two-year comprehensive mapping mission to better understand the expansion of the universe.

The expansion of the universe remains an important subject of scientific research. While it is widely accepted based on observational as well as theoretical evidence, the particulars of how and why the expansion is occurring remain open to debate. Given the importance of the expansion rate on the general understanding of the universe's past and future, investigation of the issue continues to be a leading topic among astronomers.

Principal terms

CEPHEID VARIABLE STAR: a variable star whose period of variation is related to its absolute brightness (luminosity); used to measure distances to star clusters and galaxies

DOPPLER SHIFT: a change in the observed frequency, or pitch, of sound and electromagnetic waves because of the relative motion of the source and observer; applies to the shift in the wavelength of light caused by the motion of source and observer

HUBBLE'S LAW: the relation that the spectral redshift of light from a distant galaxy is a measure of its velocity, which is dependent on the galaxy's distance

LIGHT-YEAR: an astronomical unit of distance equal to the distance light travels in one year, 9,460 billion kilometers or or 5,880 billion miles

MILKY WAY: the bright, dense band of stars seen overhead on a summer's night; the name for the galaxy in which Earth is located

PARALLAX: an apparent shift in the position of a star in the sky because of a change in the location of its observer; used to measure the distance to nearby stars

REDSHIFT: the shift in the spectrum of light from a star or galaxy toward the lower-frequency, or red, end of the spectrum, caused by the velocity of the galaxy moving away from the observer

SPECTRUM: the component wavelengths of energy generated by a star, generally referring to the visible wavelengths (violet to red); includes the distinct wavelengths (colors) absorbed by heated gases at a star's surface

Bibliography

Andreoli, Claire, et al. "New Hubble Constant Measurement Adds to Mystery of Universe's Expansion Rate." NASA, 17 July 2019, www.nasa.gov/feature/goddard/2019/new-hubble-constant-measurement-adds-to-mystery-of-universe-s-expansion-rate. Accessed 25 Feb. 2025.

Candanosa, Roberto Molar. "Webb Telescope's Largest Study of Universe Expansion Confirms Challenge to Cosmic Theory." The HUB, Johns Hopkins University, 9 Dec. 2024, hub.jhu.edu/2024/12/09/webb-telescope-hubble-tension-universe-expansion. Accessed 19 Feb. 2025.

Conover, Emily. "Scientists Still Can't Agree on the Universe's Expansion Rate." ScienceNews, 16 July 2019, www.sciencenews.org/article/universe-expansion-rate-mystery. Accessed 25 Feb. 2025.

Eddington, Arthur. The Expanding Universe. Reprint. Cambridge UP, 1987.

Gohd, Chelsea. "What Is Dark Energy? Inside Our Accelerating, Expanding Universe." NASA, 5 Feb. 2024, science.nasa.gov/universe/the-universe-is-expanding-faster-these-days-and-dark-energy-is-responsible-so-what-is-dark-energy. Accessed 25 Feb. 2025.

Hawking, Stephen W. A Brief History of Time. Bantam Books, 1988.

Pasachoff, Jay M. Astronomy: From the Earth to the Universe. Saunders College Publishing, 1991.

Rodriguez, Brandon. "Exploring the Mystery of Our Expanding Universe." NASA’s Jet Propulsion Laboratory, 11 Oct. 2024, www.jpl.nasa.gov/edu/resources/teachable-moment/exploring-the-mystery-of-our-expanding-universe. Accessed 25 Feb. 2025.

Siegel, Ethan. "This Is Why We Aren't Expanding, Even If the Universe Is." Forbes, 19 Feb. 2019, www.forbes.com/sites/startswithabang/2019/02/19/this-is-why-we-arent-expanding-even-if-the-universe-is. Accessed 25 Feb. 2025.

Weaver, Donna, et al. "Mystery of the Universe's Expansion Rate Widens with New Hubble Data." NASA, 25 Apr. 2019, www.nasa.gov/feature/goddard/2019/mystery-of-the-universe-s-expansion-rate-widens-with-new-hubble-data. Accessed 25 Feb. 2025.

"Webb & Hubble Confirm Universe’s Expansion Rate." European Space Agency, 3 Nov. 2024, www.esa.int/Science_Exploration/Space_Science/Webb/Webb_Hubble_confirm_Universe_s_expansion_rate. Accessed 25 Feb. 2025.

"What Does It Mean When They Say the Universe is Expanding?" Everyday Mysteries, Library of Congress, 19 Nov. 2019, www.loc.gov/everyday-mysteries/astronomy/item/what-does-it-mean-when-they-say-the-universe-is-expanding. Accessed 25 Feb. 2025.