How Big is the the Universe?

Our planet, Earth, is one of eight planets orbiting the sun. The sun is merely one of many hundreds of billions of stars in our galaxy known as the Milky Way. And the Milky Way is merely one of many trillions of galaxies in the known universe. The universe is so large that it is difficult, if not impossible, to determine its size. But let’s try and do exactly that. Let’s dig a little deeper and see if we can figure out just how big it really is.  Read More →

Newborn Stars and their Effect on the Universe

Star Cluster

When galaxies form new stars, they sometimes do so in frantic episodes of activity known as starbursts. These events were commonplace in the early Universe, but are rarer in nearby galaxies. Read More →

Using Black Holes to Measure the Universe’s Rate of Expansion

Black Holes

A few years ago, researchers revealed that the universe is expanding at a much faster rate than originally believed — a discovery that earned a Nobel Prize in 2011. But measuring the rate of this acceleration over large distances is still challenging and problematic, says Prof. Hagai Netzer of Tel Aviv University’s School of Physics and Astronomy.

Now, Prof. Netzer, along with Jian-Min Wang, Pu Du and Chen Hu of the Institute of High Energy Physics of the Chinese Academy of Sciences and Dr. David Valls-Gabaud of the Observatoire de Paris, has developed a method with the potential to measure distances of billions of light years with a high degree of accuracy. The method uses certain types of active black holes that lie at the center of many galaxies. The ability to measure very long distances translates into seeing further into the past of the universe — and being able to estimate its rate of expansion at a very young age.

Published in the journal Physical Review Letters (citation below), this system of measurement takes into account the radiation emitted from the material that surrounds black holes before it is absorbed. As material is drawn into a black hole, it heats up and emits a huge amount of radiation, up to a thousand times the energy produced by a large galaxy containing 100 billion stars. For this reason, it can be seen from very far distances, explains Prof. Netzer.

Solving for unknown distances

Using radiation to measure distances is a general method in astronomy, but until now black holes have never been used to help measure these distances. By adding together measurements of the amount of energy being emitted from the vicinity of the black hole to the amount of radiation which reaches Earth, it’s possible to infer the distance to the black hole itself and the time in the history of the universe when the energy was emitted.

Getting an accurate estimate of the radiation being emitted depends on the properties of the black hole. For the specific type of black holes targeted in this work, the amount of radiation emitted as the object draws matter into itself is actually proportional to its mass, say the researchers. Therefore, long-established methods to measure this mass can be used to estimate the amount of radiation involved.

The viability of this theory was proved by using the known properties of black holes in our own astronomical vicinity, “only” several hundred million light years away. Prof. Netzer believes that his system will add to the astronomer’s tool kit for measuring distances much farther away, complimenting the existing method which uses the exploding stars called supernovae.

Illuminating “Dark Energy”

According to Prof. Netzer, the ability to measure far-off distances has the potential to unravel some of the greatest mysteries of the universe, which is approximately 14 billion years old. “When we are looking into a distance of billions of light years, we are looking that far into the past,” he explains. “The light that I see today was first produced when the universe was much younger.”

One such mystery is the nature of what astronomers call “dark energy,” the most significant source of energy in the present day universe. This energy, which is manifested as some kind of “anti-gravity,” is believed to contribute towards the accelerated expansion of the universe by pushing outwards. The ultimate goal is to understand dark energy on physical grounds, answering questions such as whether this energy has been consistent throughout time and if it is likely to change in the future.

Source: American Friends of Tel Aviv University


Wang, J., Du, P., Valls-Gabaud, D., Hu, C., & Netzer, H. (2013). Super-Eddington Accreting Massive Black Holes as Long-Lived Cosmological Standards Physical Review Letters, 110 (8) DOI: 10.1103/PhysRevLett.110.081301

Dark Galaxies of the Early Universe

Dark galaxies are small, gas-rich galaxies in the early Universe that are very inefficient at forming stars. They are predicted by theories of galaxy formation and are thought to be the building blocks of today’s bright, star-filled galaxies. Astronomers think that they may have fed large galaxies with much of the gas that later formed into the stars that exist today.

Because they are essentially devoid of stars, these dark galaxies don’t emit much light, making them very hard to detect. For years astronomers have been trying to develop new techniques that could confirm the existence of these galaxies. Small absorption dips in the spectra of background sources of light have hinted at their existence. However, this new study marks the first time that such objects have been seen directly.

“Our approach to the problem of detecting a dark galaxy was simply to shine a bright light on it.” explains Simon Lilly (ETH Zurich, Switzerland), co-author of the paper. “We searched for the fluorescent glow of the gas in dark galaxies when they are illuminated by the ultraviolet light from a nearby and very bright quasar. The light from the quasar makes the dark galaxies light up in a process similar to how white clothes are illuminated by ultraviolet lamps in a night club.” [1]

The team took advantage of the large collecting area and sensitivity of the Very Large Telescope (VLT), and a series of very long exposures, to detect the extremely faint fluorescent glow of the dark galaxies. They used the FORS2 instrument to map a region of the sky around the bright quasar [2] HE 0109-3518, looking for the ultraviolet light that is emitted by hydrogen gas when it is subjected to intense radiation. Because of the expansion of the Universe, this light is actually observed as a shade of violet by the time it reaches the VLT. [3]

“After several years of attempts to detect fluorescent emission from dark galaxies, our results demonstrate the potential of our method to discover and study these fascinating and previously invisible objects,” says Sebastiano Cantalupo (University of California, Santa Cruz), lead author of the study.

The team detected almost 100 gaseous objects which lie within a few million light-years of the quasar. After a careful analysis designed to exclude objects where the emission might be powered by internal star-formation in the galaxies, rather than the light from the quasar, they finally narrowed down their search to 12 objects. These are the most convincing identifications of dark galaxies in the early Universe to date.

The astronomers were also able to determine some of the properties of the dark galaxies. They estimate that the mass of the gas in them is about 1 billion times that of the Sun, typical for gas-rich, low-mass galaxies in the early Universe. They were also able to estimate that the star formation efficiency is suppressed by a factor of more than 100 relative to typical star-forming galaxies found at similar stage in cosmic history. [4]

“Our observations with the VLT have provided evidence for the existence of compact and isolated dark clouds. With this study, we’ve made a crucial step towards revealing and understanding the obscure early stages of galaxy formation and how galaxies acquired their gas”, concludes Sebastiano Cantalupo.

The MUSE integral field spectrograph, which will be commissioned on the VLT in 2013, will be an extremely powerful tool for the study of these objects.


Sebastiano Cantalupo, Simon J. Lilly, & Martin G. Haehnelt (2012). Detection of dark galaxies and circum-galactic filaments fluorescently illuminated by a quasar at z=2.4 Monthly Notices of the Royal Astronomical Society :


[1] Fluorescence is the emission of light by a substance illuminated by a light source. In most cases, the emitted light has longer wavelength than the source light. For instance, fluorescent lamps transform ultraviolet radiation — invisible to us — into optical light. Fluorescence appears naturally in some compounds, such as rocks or minerals but can be also added intentionally as in detergents that contain fluorescent chemicals to make white clothes appear brighter under normal light.

[2] Quasars are very bright, distant galaxies that are believed to be powered by supermassive black holes at their centres. Their brightness makes them powerful beacons that can help to illuminate the surrounding area, probing the era when the first stars and galaxies were forming out of primordial gas.

[3] This emission from hydrogen is known as Lyman-alpha radiation, and is produced when electrons in hydrogen atoms drop from the second-lowest to the lowest energy level. It is a type of ultraviolet light. Because the Universe is expanding, the wavelength of light from objects gets stretched as it passes through space. The further light has to travel, the more its wavelength is stretched. As red is the longest wavelength visible to our eyes, this process is literally a shift in wavelength towards the red end of the spectrum — hence the name ‘redshift’. The quasar HE 0109-3518 is located at a redshift of z = 2.4, and the ultraviolet light from the dark galaxies is shifted into the visible spectrum. A narrow-band filter was specially designed to isolate the specific wavelength of light that the fluorescent emission is redshifted to. The filter was centered at around 414.5 nanometres in order to capture Lyman-alpha emission redshifted by z=2.4 (this corresponds to a shade of violet) and has a bandpass of only 4 nanometres.

[4] The star formation efficiency is the mass of newly formed stars over the mass of gas available to form stars. They found these objects would need more than 100 billion years to convert their gas into stars. This result is in accordance with recent theoretical studies that have suggested that gas-rich low-mass haloes at high redshift may have very low star formation efficiency as a consequence of lower metal content.

Source: ESO

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Understanding Black Holes – The Basics

Black holes are frequently mentioned as a sort of metaphor. In reality, black holes are generally understood to be large, empty voids in space where light cannot escape. It’s almost as if objects and energy are sucked into them. Though this is the basic premise behind black holes, they are much more complex objects that can tell us a number of facts about our universe and how it began.

Definition of Black Holes

Black holes are the last evolutionary stage in the lives of enormous stars that were once 10 to 15 times the size of our own sun. At the end of their lives, they may explode in a gigantic “supernova” event that scatters matter, but leaves behind a cold remnant of the star that collapses in on itself. This is the budding black hole that begins to suck in matter and light. However, light and matter must pass so close to black hole so that they cannot escape. This point is called the “event horizon.”

The Study of Black Holes

Black holes have been theorized since the late 1700’s. Einstein’s theories also predicted the existence of black holes. However, it wasn’t until 1967 that physicist John Wheeler began referring to these phenomena as “black holes.” Though no one has ever seen one, these black holes gave way to much scientific study in recent decades.

What Black Holes Tell Us

The study of black holes has demonstrated how the universe can hide much of its matter. This fact helps to account for all the missing matter that falls outside the mathematical computations about the universe. Black holes not only probably existed when our galaxy was formed, but also aided in the galaxy’s formation. The study of black holes also suggests that all matter that exists can become a black hole if compressed to zero volume and thus, infinite density.

Facts About Black Holes

  • At the center of each black hole is a point where the laws of physics break down and space-time cease to exist.  This point is called a “singularity.”
  • Black holes vary in size depending on the size of the matter within it. No one has actually seen a black hole because light does not escape from a black hole. However, the dust and gas clouds that swirl around black holes emit radiation that can be detected.
  • Black holes can suck up other black holes when they are in close proximity to each other, usually with the larger ones devouring smaller ones. Black holes can also circle each other in a swirling motion.
  • As black holes age, they gain more mass because they gain more matter over time. Black holes will eventually disintegrate over trillions of years.
  • The nearest black hole in 1,600 light years away. It is called V4641 Sgr and is a rare type of black hole called a micro quasar. This black hole is located in the Sagittarius arm of the Milky Way.
  • Black holes have no temperature, but objects about to enter the black hole are heated to millions of degrees before they disappear. They also emit x-rays.
  • Black holes do not actually suck things into them. The objects simply fall into the black hole and disappear due to the dense gravity of the hole.
  • Black holes are the simplest objects in the universe and can be described completely by their mass, spin rate, and electrical charge.

Of course all of this could change tomorrow if someone were to somehow disprove the theories behind black holes. Many people debate whether they even exist or not however scientific research in recent years has generally reinforced their existence and role within the universe.

Image Credit: XMM-Newton/ESA/NASA

String Theory & Quantum Gravity: Simplified

Physics is the study of matter and how it moves through spacetime. For many years, physicists have attempted to understand how the forces of the gravity, which regulates large bodies like the planets, and those of quantum mechanics, which regulates small particles like atoms, can be explained in a “unified theory of everything.” String theory attempted to make sense of these various forces in a mathematically coherent way.  Problems with string theory spawned an alternate model called the loop quantum gravity theory which I will cover as well in today’s post. Read More →