It is natural to think of our universe as being composed of the planets, stars, nebulae, and galaxies visible in our telescopes. In addition to these bright objects, there are non-glowing components like dark gas clouds, burned-out stars, and black holes. In sum, these comprise what we’ll call the “normal” matter in the cosmos.
Many decades ago, astronomers found evidence for another component: dark matter. By measuring the motions of galaxies, astronomers could estimate the total amount of mass required to create those motions — a “dynamical mass estimate.” They could also add up all the mass they could detect — a “luminous mass estimate.” In almost all cases, the dynamical mass estimate was larger than the luminous mass estimate by factors of five to ten. In other words, the amount of mass astronomers could actually “see” was only about 10 to 20 percent of the mass that actually existed.
Even the most generous estimates for non-luminous normal matter — factoring in all the dim objects we cannot see — could not account for such a discrepancy. Hence, astronomers concluded that dark matter must exist. This dark matter is not made of the same stuff as normal matter, and is so far only evident by the gravitational forces it exerts.
Hubble has provided striking examples of dark matter and has helped to greatly refine measurements of dark matter. In large clusters of galaxies, Hubble’s resolution is key to measuring an effect known as gravitational lensing (see the article entitled “Can Hubble ‘See’ Gravity?”). From gravitational lensing observations, scientists can create a map of the mass distribution in the galaxy cluster.
In one cluster, Cl 0024+17 (pictured above), the mass map shows a wide ring around the cluster that appears to have been produced from a collision between two gigantic clusters. In another collision occurring in the Bullet Cluster of galaxies, the dark matter has streamed through the collision, while the diffuse normal matter, as seen in X-rays by the Chandra X-ray Observatory, has smashed together, heated up, and remained at the location of impact. These mass maps show both the details of how the dark matter is distributed and that it does not behave in the same way as normal matter.
Unfortunately, Hubble observations have not yet helped determine the composition of dark matter. Particle physics has provided a number of ideas that continue to be studied.
A similar shroud of mystery encompasses a more recently discovered cosmic component. In the late 1990s, astronomers found evidence that the expansion of the universe was not slowing down as expected. Instead, the expansion speed was increasing. Something had to be powering this accelerating universe, and, in part due to its unknown nature, it was called dark energy.
Hubble played an important role in verifying, characterizing, and constraining dark energy. Both Hubble and ground-based observations had measured stellar explosions, called supernovae, to measure accurate distances to galaxies. A galaxy located a billion light-years away provides a data point for the universe as it was a billion years ago. Meanwhile, as the universe expands, the light traveling to Earth from distant galaxies (and their supernovae) is “stretched out” to longer wavelengths — a phenomenon called “cosmological redshift.” The cosmological redshifts of galaxies at different distances provides a history of the expansion of the universe over time.
However, only Hubble had the resolution to extend these observations to very distant galaxies. The discovery of supernova 1997ff, located about 10 billion light-years away, provided a crucial piece of evidence. In a universe with dark matter, the expansion starts out in the standard manner, with the expansion speed slowing down for billions of years. Only about halfway into the Universe’s history -- several billions of years ago -- does dark energy become dominant and the expansion accelerates. While ground-based studies had measured this accelerating period, Hubble’s observation of 1997ff stretched back to the decelerating part of the expansion. For skeptical astronomers, observing this evidence of a shift between two different eras of the universe – a change from a decelerating universe to an accelerating universe -- provided the final evidence that dark energy exists.
Hubble continues to explore the nature of dark energy with observations such as the Great Observatories Origins Deep Survey (GOODS), structured to help uncover distant supernovae. The 42 supernovae found by Hubble not only solidified the conclusions about dark energy but also began to constrain some of its possible explanations. Later Hubble results identified how early in the universe dark energy began to influence the expansion as well as constrained the current expansion rate.
The view that emerged was that dark energy was consistent with the slow, steady force of Einstein’s “cosmological constant,” a concept that the physicist had initially introduced into his equations to prevent his theoretical universe from expanding, then later retracted when the expansion of the universe was discovered. Dark energy turned what Einstein had once called his “greatest mistake” into standard feature of modern cosmology. The discovery of dark energy won the Nobel Prize in Physics in 2011.
Astronomers now know that there is much more to the universe than meets the eye. The luminous and non-luminous normal matter makes up about 4% of the total mass and energy density of the universe. Dark matter comprises another 24% of the total, while dark energy dominates with about 72%. Most of the universe is unknown and only indirectly detected. We can see its effects on galaxies and the expansion of the universe, but we have yet to identify the underlying source. That may seem unsettling, but to a scientist, it is exciting. There are yet more great mysteries to be explored and solved.
The era of the universe called the “Dark Ages” is as mysterious as its name implies.
Shortly after the Big Bang, our universe was filled with a glowing plasma, or ionized gas.
As the universe cooled and expanded, electrons and protons began to bind together to form neutral hydrogen atoms. The last of the light from the Big Bang escaped (becoming what we now detect as the Cosmic Microwave Background). The universe would have been a dark place, with no sources of light to reveal this cooling, neutral hydrogen gas.
Some of that gas would have begun coalescing into dense clumps, pulled together by gravity. As these clumps grew larger, they would become stars and eventually galaxies. Slowly, starlight would begin to shine in the universe.
Eventually, as the early stars grew in numbers and brightness, they would have emitted enough ultraviolet radiation to “reionize” the hydrogen, or turn it back into protons and electrons. At this point, the light from star and galaxy formation could travel freely across space and illuminate the universe.
The universe's first stars, believed to be 30 to 300 times as massive as our Sun and millions of times as bright, would have burned for only a few million years before dying in tremendous explosions, or “supernovae.” These explosions spewed the recently manufactured chemical elements of stars outward into the universe before the expiring stars collapsed into black holes or dim, cinder-like cores.
Astronomers know the universe became reionized because when they look back at quasars — incredibly bright objects thought to be powered by supermassive black holes — in the distant universe, they don’t see the dimming of their light that would occur through a fog of neutral hydrogen gas. They find clouds of hydrogen, but almost no detectable intergalactic medium of neutral hydrogen, meaning the gas was at some point reionized. Exactly when this occurred is one of the questions Webb will help answer, by looking for glimpses of very distant objects still dimmed by neutral hydrogen gas.
Much remains to be uncovered about the time of reionization. The universe right after the Big Bang would have consisted of hydrogen, helium, and a small amount of lithium. But the stars we see today also contain heavier elements — elements that are created inside stars. So how did those first stars form from such limited ingredients? Webb will not be able to see the very first stars of the Dark Ages, but it’ll witness the generation immediately following, and analyze the kinds of materials they contain.
Webb’s ability to see the infrared light from the most distant objects in the universe will allow it to truly identify the sources that gave rise to reionization. For the first time, we will be able to see the stars and quasars that unleashed enough energy to illuminate the universe again.
Webb will also show us how early galaxies formed from those first clumps of stars. Scientists suspect the black holes born from the explosion of the earliest stars devoured gas and stars around them, becoming the extremely bright objects called “mini-quasars.” The mini-quasars, in turn, may have grown and merged to become the huge black holes found in the centers of present-day galaxies. Webb will try to find and understand these supernovae and mini-quasars to put theories of early universe formation to the test. Do all early galaxies have these “active galactic nuclei” (AGN), as they’re known, or only some? These regions give off infrared light as the gas around them cools, allowing Webb to glean information about how AGN in the early universe work — how hot they are, for instance, and how dense.
Webb will show us whether the first galaxies formed along lines and webs of dark matter, as expected, and when. Right now we know the first galaxies formed anywhere from 378,000 years to 1 billion years after the Big Bang. Many models have been created to explain which era gave rise to galaxies, but Webb will pinpoint the precise time period.
Hubble is known for its deep-field images, which capture slices of the universe throughout time. But these images stop at the point beyond which Hubble’s vision cannot reach. Webb will fill in the gaps in these images, extending them back to the Dark Ages. Working together, Hubble and Webb will help us visualize much more of the universe than we ever have before, creating for us an unprecedented picture that stretches from the current day to the beginning of the recognizable universe.
Gravity is the familiar force of nature responsible for the diverse motions of a baseball thrown high into the air, a planet orbiting a star, or a star orbiting within a galaxy. Astronomers have long observed such motions and deduced the amount of gravity, and therefore the amount of matter, present in the planet, star, or galaxy. When taken to the extreme, gravity can also create some intriguing visual effects that are well suited to Hubble’s high-resolution observations.
Einstein’s general theory of relativity expresses how very large mass concentrations distort the space around them. Light passing through that distorted space is re-directed, and can produce a variety of interesting imagery. The bending of light by gravity is similar to the bending of light by a glass lens, hence we call this effect “gravitational lensing.”
The simplest type of gravitational lensing is called “point source” lensing. There is a single concentration of matter at the center, such as the dense core of a galaxy. The light of a distant galaxy is re-directed around this core, often producing multiple images of the background galaxy (see image accompanying this article). When the lensing approaches perfect symmetry, a complete or almost-complete circle of light is produced, called an “Einstein ring.” Hubble observations have helped to greatly increase the number of Einstein rings known to astronomers.
More complex gravitational lensing arises in observations of massive clusters of galaxies. While the distribution of matter in a galaxy cluster generally does have a center, it is never circularly symmetric and can be significantly lumpy. Background galaxies are lensed by the cluster, with their images often appearing as short, thin “lensed arcs” around the outskirts of the cluster. Hubble’s images of galaxy clusters, such as Abell 2218 and Abell 1689, showed the large number and detailed distribution of these lensed images throughout massive galaxy clusters.
These lensed images also act as probes of the matter distribution in the galaxy cluster. Astronomers can measure the motions of the galaxies within a cluster to determine the total amount of matter in the cluster. The result indicates that the most of the matter in a galaxy cluster is not in the visible galaxies and does not emit light, and is thus called “dark matter.” The distribution of lensed images reflects the distribution of all matter, both visible and dark. Hence, Hubble’s images of gravitational lensing have been used to create maps of dark matter in galaxy clusters.
In turn, a map of the matter in a galaxy cluster helps provide better understanding and analysis of the gravitationally lensed images. A model of the matter distribution can help identify multiple images of the same galaxy or be used to predict where the most distant galaxies are likely to appear in a galaxy cluster image. Astronomers work back and forth between the gravitational lenses and the cluster matter distribution to improve our understanding of both.
On top of it all, gravitational lenses extend Hubble’s view deeper into the universe. Very distant galaxies are very faint. Gravitational lensing not only distorts the image of a background galaxy, it can amplify its light. Looking through a lensing galaxy cluster, Hubble can see fainter and more distant galaxies than otherwise possible. It is like having an extra lens that is the size of the galaxy cluster. The Frontier Fields project has examined multiple galaxy clusters, measured their lensing and matter distribution, and identified a collection of these most distant galaxies.
While the effects of normal gravity are measurable in the motions of objects, the effects of extreme gravity are visible in images of gravitational lensing. The diverse, lensed images of crosses, rings, arcs, and more are both intriguing and informative. Gravitational lensing probes the distribution of matter in galaxies and clusters of galaxies, and enables observations of the distant universe. Hubble’s data will also provide a basis and guide for the future James Webb Space Telescope, whose infrared observations will push yet farther into the cosmos.
The distorted imagery of gravitational lensing often is likened to the distorted reflections of funhouse mirrors, but don’t take that comparison too far. Hubble’s images of gravitational lensing provide a wide range of serious science.
When we look out into space, we are also looking back in time. The light arriving at our telescope from the farthest objects in the universe is light that left those objects billions of years ago. Hence, we see those distant galaxies as they appeared long ago. The most-distant galaxies look strange — smaller, irregular, lacking clearly defined shapes.
No telescope before Hubble had the resolution and sensitivity to see these distant galaxies. Astronomers were curious if they could truly detect these faint far-away galaxies. They pointed Hubble at what appeared to be a nearly empty tiny patch of sky and let it soak up light for a cumulative exposure of about 10 days (about 860,000 seconds). The area of the sky imaged was about the size of a pinhole seen at arm’s length. The astronomers were taking a risk — most Hubble observations take just hours. The proposed time for this ‘deep field’ could have been used for many other investigations. It was possible the objects the astronomers were looking for would be too faint or small for even Hubble to see.
But the results turned up a treasure trove: 3,000 galaxies, large and small, shapely and amorphous, shining in the depths of space. The stunning image was called the Hubble Deep Field (HDF). A separate series of observations in the southern hemisphere gave similar results in what was dubbed the Hubble Deep Field South.
New technology enables new ground-breaking science. Over the intervening years, Hubble was serviced by astronauts and given new instruments that were more sensitive and allowed for many additional colors of light to be captured. With the imaging of the Hubble Ultra Deep Field in 2004 (HUDF-2004), Hubble began a new examination of the deep universe. The HUDF-2004 captured a deeper view of the universe in visible light. Along with an increase in observing time from the original HDF (approximately a million seconds), the increased sensitivity of the new camera on Hubble allowed for the detection of about 10,000 galaxies in the HUDF-2004 – about three times the number observed in the original deep field.
After Hubble’s last servicing mission in 2009, additional instruments with enhanced sensitivity were added. This mission gave Hubble the ability to see infrared light -- light waves longer than visible light and invisible to the human eye. The expanding universe causes space itself to stretch, which in turn stretches waves of light as they travel through the expanding space. The longer a light wave travels, the more it is stretched. Light waves from the most distant galaxies have been traveling through space for so long that they have been stretched into a longer region of wavelengths: infrared light. Astronomers did brand-new observations of the HUDF in 2009 (HUDF-2009) utilizing Hubble’s new view of infrared light. By adding this enabling technology to Hubble, astronomers are now able to capture even more distant views of the universe. These distant views are discovering young galaxies at the earliest times yet probed by Hubble.
When it comes to Hubble’s view of the past, the culmination of our knowledge of the deep universe came from yet deeper infrared observations in 2012 (HUDF-2012), together with the release of the eXtreme Deep Field (XDF). In the XDF, astronomers combined all Hubble observations, taken over 10 years, of a slightly smaller portion of the original HUDF-2004 field to match the size of the infrared images. The XDF includes data from the HUDF-2004, HUDF-2009, and the HUDF-2012 imaging campaigns, along with imaging data of the same region from many other programs. The total observing time of the data included in the XDF is around two million seconds. The accumulated infrared light used in both the HUDF-2012 and XDF images gives this region of the sky the distinction of being Hubble’s deepest view of the universe. The latest observations in Hubble’s HUDF program were used to create the HUDF-2014 (or UV-UDF) image, which includes added ultraviolet light that helps give astronomers added information about star formation within the galaxies.
And yet, there is even more to the story. Hubble is also pioneering deep surveys, which do not go quite as deep as the HUDFs or XDF, but instead observe larger portions of the sky. The benefits of the deep surveys, like the Great Observatories Origins Deep Survey (GOODS), the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), and the Cluster Lensing And Supernova survey with Hubble (CLASH), are that they provide larger numbers of galaxies, and other phenomena, to study.
Hubble’s next great deep observing campaign is called Frontier Fields. Frontier Fields combines the power of Hubble with the natural magnifying power of huge clusters of galaxies. The gravity of the galaxy clusters bends and magnifies the light of more-distant galaxies behind them, giving the light a boost that makes it observable by Hubble. Using this technique, Hubble is expected to obtain even deeper views of the universe and find even more distant galaxies. These furthest galaxies are the galaxies that the infrared-observing James Webb Space Telescope will routinely find after it launches later this decade.
Watch throughout the year for more articles on Hubble's 25 years of discovery.