The Optical Extragalactic Background Light — A Window to the Early Universe

Before providing a glimpse into my research I would like to first say that my summer spent at RIT has been a wonderful experience and that I consider myself incredibly lucky to have had this opportunity.When I was applying for research positions this past winter I knew that I was a leg behind the competition being that I was just coming out of freshman year and had no prior research experience. Fortunately for me, however, Dr. Roger Dube here at RIT accepted my application and has been my research mentor this summer. Thank you to Dr. Dube, RIT, the NSF, my great professors at Trinity College, and everyone else who has made this experience possible.

I have been investigating the extragalactic background light (EBL) in the optical wavelengths. The EBL (assume all uses of EBL are in reference to the optical EBL) has been a topic of research for several decades as it may contain valuable information about the earliest epochs of the Universe and provide us with a better understanding of cosmological processes.

Understanding the EBL could particularly bring us closer to solving the Missing Mass Problem. The problem is that the current inventory of baryonic mass in the Universe accounts for only a fraction of the mass required to explain observed gravitational forces within and between galaxies. In order to completely comprehend the nature and fate of the Universe, we must precisely account for its mass content because the interplay between gravity and dark energy is an interesting relationship. Dark energy is repulsive to gravity and responsible for the expansion of the Universe. In other words, dark energy is the force that has been pushing everything in the Universe apart and away from each other since the Big Bang. The ratio of dark energy to the Universe’s mass density can provide us with the necessary information to deduce the destiny of our Universe — whether it will continue expanding forever and may eventually tear apart (The Big Rip) or that gravity will overcome dark energy to cause the Universe to collapse into itself (The Big Crunch). This relationship is similar to that of a rocket launching off of a planet’s surface. If the rocket’s propulsion force (comparable to dark energy in the Universe) is greater than the gravity of the planet then it will launch into space and virtually escape the planet’s gravitational attraction. However, the planet’s mass and density may be necessarily great such that gravity overcomes the rocket’s propulsion and pulls it crashing back down to the surface.

The observed parameters for the matter density of the Universe and the Cosmological Constant (a measure of dark energy) suggest that gravity is not great enough to overcome expansion (along with other problems and theories that are interesting to cosmology such as Inflation, the Horizon Problem, and the Flatness Problem which I shouldn’t get into here). The value of the Cosmological Constant is widely accepted because it is consistently verified by observation of both the cosmic microwave background (CMB) and a method utilizing the redshift data from supernovae, but what if we have underestimated the mass density of our Universe?

We can use the information stored in the light we receive from distant objects in space to decipher mass content by utilizing the mass-to-light ratio which relates the amount of light emitted by a source with its mass. The science of photometry measures photon flux and is used by astrophysicists to quantify the amount of light received by an instrument directed at a source. Photometry is a very useful tool and there is software available that makes photometry relatively user-friendly to measure the light from bright sources such as galaxies and stars. Understandably so, however, the brightness of a source relative to its surroundings and the difficulty to quantify its brightness are inversely related; fainter sources of light are harder to measure. As I will explain, the EBL is incredibly dim and has evaded precise measurement. However, the EBL contains valuable information about a sea of primeval galaxies in our Universe that cannot be otherwise directly observed; a proper measurement of the EBL could blossom cosmological speculation into Universal understanding.

The EBL is just what it sounds like — background light. The deepest optical images of the Universe, provided by the Hubble Space Telescope (HST), aren’t perfect, and the furthest reaches of our Universe, which are also the youngest, still remain unresolvable by even the best available technology. The deepest resolved point sources on record seem to be faint galaxies around 13 billion light years away which means that we are observing these galaxies as they were in the first 1 billion years of our Universe. The regions between these galaxies, the extragalactic space, are very dark and appear empty, but this is a result of bright foreground emissions dominating the focal plane of these images, absorption/ scattering of light, and redshift of light out of the optical wavelengths rather than true emptiness. The EBL is the light emitted by unresolvable primeval galaxies, beyond the deepest observable galaxies, which can be measured by the brightness of these very dim extragalactic regions. The EBL holds information about the mass content of the Universe that we cannot yet directly measure. Unfortunately, an absolute measurement of the EBL is about as subtle as the squeak of a mouse in a crowded cafeteria and such a measurement has eluded scientists for decades.

My mentor, Dr. Roger Dube, was one of the pioneers who started the movement to measure the absolute EBL. He applied a novel technique to measure the brightness of extragalactic regions in his 1979 paper that he published while pursuing his PhD at Princeton (http://adsabs.harvard.edu/cgi-bin/bib_query?1979ApJ…232..333D). However, he was only able to provide an upper limit on the value of the EBL due to large sources of uncertainties. Since Dube, there have been several attempts to determine the absolute value of the EBL but each one has similarly fallen short and been forced to settle with presenting upper and lower limits rather than a definitive value. In one of the most recent studies, Rebecca Bernstein of Caltech claimed the first detection of the EBL (https://ned.ipac.caltech.edu/level5/Sept01/Bernstein/BFM_contents.html). After several revisional papers, however, she too concluded that her measurements represented limits to the EBL.

The fundamental problem underlying the detection of the EBL is that the expected value, the signal, is minuscule compared to the foreground sources of light, or the noise, that must be accounted for and removed. This low signal-to-noise ratio coupled with experimental procedures lacking the methods and/or the technology to produce significant results is responsible for prior attempts falling short to uncover an absolute measurement of the EBL. Quantitatively speaking, Bernstein’s study minimized necessary noise subtractions by only using images with sources dimmer than 23 AB mag — a measure of brightness much fainter than distinguishable by most telescopes, let alone the naked eye — yet her signal-to-noise ratios for the lower limits on the EBL in her two optical filters were 0.004 and 0.005. The lower limit signal only represents 0.4-0.5% of the total light intensity contributing to the noise in the data. Imperfect models for zodiacal light(ZL), atmospheric light scattering, diffuse galactic light (DGL) scattering, and instrumental uncertainties further compound error bounds resulting in uncertainties of nearly equal magnitude to the measured “detection” of the EBL. In order to provide an absolute measurement of the EBL, which Bernstein’s work suggests may require subtracting noise that could be near 200 times greater than the signal itself, the sources of uncertainty and error that plagued the work of Dube and Bernstein must be further corrected.

The beginning of my 10-week research program began with Dr. Dube providing me with the paper he published in 1979, a review paper published in the 80’s on EBL, my own lab, he helped me understand the topic by discussing it with me as I had no astronomy or astrophysics background, and then he let me off the leash to take on my project. As I write this I am finishing up the last 2 weeks of research before my symposium presentation and the long road leading to this point included: reading 4 textbooks, countless research papers, over 1,000 pages of space telescope instrumental and data handbooks, learning a handful of photometry, spectroscopy, and imaging software programs, hours of digging through data archives, learning Python (my first computer programming language), and mastering the art of cooking eggs and cheddar cheese. Looking back on this summer I’m proud of what I’ve accomplished, consider myself lucky to have met such incredible people, and have made memories that will last a lifetime. Moving forward, I’m excited to continue working with Dr. Dube as we plan to secure grants and space telescope time to employ an innovative new strategy we have designed. This new method could prospectively result in the most precise absolute measurement of the optical extragalactic background light and provide invaluable information about the distant past and far future of our Universe.

 

 

(I have chosen to leave out the details for our strategy to measure the absolute EBL and only scratched the surface of the complications that accompany pursuing it. I hope that I have provided enough of an overview of my research and experience the summer. If not, please feel free to contact me through my email address which you can find on my About Me page.)

 

Christopher Giottonini 7/26/16

 

 

 

 

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