cosmologists

"Gravity is the reason why the Universe itself can even exist and evolve. It elevates space and time from mere pieces of scenery into central actors in the unfolding drama of reality. As we embrace gravity, we can't help but also pit ourselves against it: leaping, floating, or flying as we pursue brief moments of freedom from its command. I, for one, have been chasing gravity my entire life—seeking, like so many scientists who have come before me, to unravel its deepest mysteries."

"... we can never really shield ourselves from gravity. You can think of a for electromagnetism — where you can shield yourself from . But that is not the case for gravity. Everyone is connected through gravity."

"After the Big Bang, the universe expanded and cooled down. And we expect this expansion to gradually slow down because the universe has things like galaxies inside it, and they are attracted to each other by gravity. But in the past 25 years or so, observations have shown precisely the opposite: The expansion of the universe is speeding up. It’s accelerating. This is the concept of , and it points to something that we are missing in our description of the universe. Dark energy is sometimes seen as this mysterious, magical source of energy that accelerates the expansion of the universe. But this isn’t really the core of the problem. We can cook something up for dark energy — just as we do for dark matter — and hope we’ll detect it later. It’s not particularly satisfying, but we do this, we’ve done it before. Really, ."

"A theory of is one in which the , the particle that is believed to mediate the force of gravity, has a small mass. This contrasts with general relativity, our current best theory of gravity, which predicts that the graviton is exactly massless. In 2011, Claudia de Rham (), () and Andrew Tolley (Imperial College London) revitalised interest in massive gravity by uncovering the structure of the best possible (in a technical sense) theory of massive gravity, now known as the dRGT theory, after these authors. Claudia de Rham has now written a popular book on the physics of gravity. The Beauty of Falling is an enjoyable and relatively quick read: a first-hand and personal glimpse into the life of a and the process of discovery."

"For almost a century the theory of general relativity (GR) has been known to describe the force of gravity with impeccable agreement with observations. ... Far from a purely academic exercise, the existence of consistent alternatives to describe the theory of gravitation is actually essential to test the theory of GR. Furthermore the open questions that remain behind the puzzles at the interface between gravity/cosmology and particle physics such as the hierarchy problem, the old and the origin of the late-time acceleration of the Universe have pushed the search for alternatives to GR."

"We are at an interesting juncture in cosmology. With new methods and technology, the accuracy in measurement of the has vastly improved, but a recent tension has arisen that is either signaling or as-yet unrecognized uncertainties. Just under a century ago, Edwin Hubble revolutionized cosmology with his discovery that the . Hubble found a relationship between radial velocity and the distance to nearby galaxies, determining the proportionality constant Ho (=v/r), that now bears his name. The Hubble constant remains one of the most important parameters in cosmology. An accurate value of Ho can provide a powerful constraint on the cosmological model describing the evolution of the universe. In addition, it characterizes the expansion rate of the Universe at the current time, defines the observable size of the Universe, and its inverse sets the ."

"... after finishing my postdoc, I became a faculty member at , and then ended up being the scientific leader of this project to measure the rate at which the universe is expanding. ... when that project finished, .. we .. resolved the issue. We went from a factor of 2 uncertainty — we measured an uncertainty of 10%."

"The , which measures the , together with the total energy density of the Universe, sets the size of the observable Universe, its age, and its radius of curvature. Excellent progress has been made recently toward the measurement of the Hubble constant: a number of different methods for measuring distances have been developed and refined, and a primary project of the has been the accurate calibration of this difficult-to-measure parameter. The recent progress in these measurements is summarized, and areas where further work is needed are discussed. Currently, for a wide range of possible s, the Universe appears to have a . Combined with current estimates of stellar ages, the results favor a . They are consistent with either an , or a with a non-zero value of the ."

"is that is assumed to be absolutely stable. A seed of strange matter in a will convert the star into a . The speed at which this conversion occurs is calculated. The calculation takes into account the rate at which the and s equilibrate via s and the diffusion of strange quarks towards the conversion front. The speed is found as a function of the temperature of the star and the minimum strangeness necessary for strange matter stability. The conversion can be detected as an energy release of ∼58 MeV with different luminosities for different stages of a neutron star's evolution and as a “super-glitch” on s' frequencies."

"Being a woman is hard ... The expectations are that we’ll be gorgeous, completely organized, great mothers, great wives, and great professionals ... The pressures are huge. I was always expected to bring the cookies to meetings because I’m a woman. Like, men can’t find cookies? That’s why I say: Let’s be nicer to ourselves. Diversity is very important ... When you bring in new folks who ask new questions, you change the field and the basic sciences. Women want to do great research. But we’re asking, ‘Why do we have this rule? That artificial hierarchy?’ We can be more collaborative. And I’ll tell you this ... When men suggest we speak with deeper voices if we want to get our message across, here is what I say: Gravity does not care how deep your voice is."

"The Of Extreme Multi-Messenger Astrophysics (POEMMA) was designed as a Astrophysics probe-class mission to identify the sources of ) and observe s from extremely energetic transient sources. POEMMA consists of two identical spacecraft flying in a loose formation at 525 km altitude oriented to view a common atmospheric volume and to provide full-sky coverage for both types of messengers. Each spacecraft hosts a wide with a hybrid focal plane optimized to observe both the UV fluorescence signal from extensive air showers (EASs) and the optical signals from EASs."

"I was really interested in the basic workings of nature: Why ? What is the unified theory of everything? But I realized how many easier questions we have in astrophysics: that you could actually take a lifetime and go answer them. Graduate school at MIT showed me the way to astrophysics — how it can be an amazing route to many questions, including how the universe looks, how it functions, and even particle physics questions. I didn’t plan to study s; but every step it was, “OK, it looks promising.”"

"Scientists are very curious ... We might be curious about something that looks completely irrelevant, and it turns out to be a really amazing thing. Or it could be irrelevant. We don't know. If we knew, we wouldn't be looking at it."

"Having that bit of diabolical contrariness is a weird pleasure of being a scientist. You’re always trying to figure out, “OK, how could I be fooling myself into a wrong conclusion?” Because the more you get those things right, the more chances you have of catching the universe doing something that our brains never would’ve imagined. Scientists build out of what seems like a stance of weakness. So, one might think it’s terrible that scientists are always discovering new ways that they’re wrong, or it’s terrible that they’re only probabilistically sure of facts. But that’s really where scientists’ superpower has come from. We have been able to figure out amazing solutions to problems or surprises about the world. Much of that can be traced back to being willing to be wrong and being comfortable with finding the ways you’re wrong. And for this purpose, you want to build strong relationships with people who are going to tell you when you’re wrong, who will disagree with you, or who compete with you. They’re your best bet at figuring out where you’re making a mistake."

"Today we are able to make very precise measurements of the by measuring the distances and s of . The shift in a supernova’s due to the expansion of space gives its redshift (z) and the relation between redshift and distance is used to determine the expansion rate of the universe. Supernovae with greater redshifts, lying at greater distances, reveal the past expansion rate as their light was emitted at an epoch when the universe was younger. Supernovae Type Ia were the suitable candidate for these measurements as you need objects that are very luminous (thus can be observed even when they are very far) and highly uniform (so that intrinsic scatter doesn't blur the signal). Supernovae Type Ia are the most luminous of the common supernova types, peaking at 4 billion , and thus allowing us to look at extreme large distances."

"As we discover , we suffer from a variety of s, both in our nearby and distant searches. The most significant effect is – a selection effect which leads brightness limited searches finding brighter than average objects near their sensitivity limit. This bias is caused by the larger volume in which bright objects can be uncovered compared to their fainter counterparts. Malmquist bias errors are proportional to the square of the intrinsic dispersion of the distance method, and because SN Ia are such accurate distance indicators, these errors are quite small – approximately 2%. We use s to estimate these effects, and remove their effects from our s."

"… one does not expect a . To my mind, I was genuinely surprised because when you make a discovery of acceleration–well, what causes the acceleration? Well, we give it a name–'–but we don’t understand it yet. I would not be surprised if we don’t understand it during my lifetime. Without understanding it, I felt it wouldn’t be worthy of a Nobel Prize. The fact that it was given pretty timely–you know, I was only 44 last year–was a bit of surprise."

"Less than a century ago we had no idea that there was more to the universe than our own Milky Way. the immense size of the universe, the fact that it is expanding, the fact that it is populated with such things as – all this and more had to be discovered before we could do the work that led us to contemplate an ."

"With the as an anchor, theory converged on a standard model of the universe, which was still in place in 1998, at the time of our discovery of the . This standard model was based on the theory of general relativity, and two assumptions. assumption one was that the , and assumption two that it is composed of normal , i.e. matter whose density falls directly in proportion to the volume of space, which it occupies. Within this framework, it was possible to devise observational tests of the overall theory, as well as provide values for the fundamental constants within this model – the current expansion rate (), and the average density of matter in the universe. For this model, it was also possible to directly relate the density of the universe to the rate of cosmic deceleration and the geometry of space. it stated that the more material the faster the deceleration, that above a critical density the universe has a and below this a ."

"is about the gravity of empty space, the gravity of the vacuum. And the vacuum is a concept that we address in and quantum mechanics. Quantum mechanics is physics on microscopic scales, while Einstein's theory of general relativity ... is physics on macroscopic scales. These two theories are both great, but they don't work together. We don't have what's called a quantum theory of gravity, the way the two are united. However, dark energy actually requires you to use both of these branches of physics. So our hope is that by observing how the universe actually does physics at that interface, we will learn how to unify those."

"I think . I never would have guessed it. Even as an undergraduate, once I’d learned a little physics, I would have thought that the universe was eternal, static, and always in equilibrium. So in graduate school when I found out that the universe was expanding, I was awestruck. Then I learned if we could measure the expanding universe, the way we record the growth of a child with marks on a doorframe ... , we could determine the age of the universe and predict its ultimate fate. This was staggering! I knew this is what I wanted to do. Since that time, charting the expanding universe to determine its nature has been my passion. Though I have to add: knowing what I know now, that the , I feel like King Alfonso X of Castile who saw Ptolemy’s and reportedly said “If the Lord Almighty had consulted me before embarking on creation thus, I should have recommended something simpler.”"

"Not only does it swallow anything that comes too near it but no one lives to tell the tale ... there are footprints leading in, and no footprints leading out ... If black holes weren’t real, I think the science-fiction writers would have wanted to invent them."

"The universe is filled with thermal radiation having a current temperature of 2.75 K. Originating in the very early universe, this radiation furnishes strong evidence that the Big Bang cosmology best describes our expanding universe from an incredibly hot, compacted early stage until now. The model can be used to extrapolate our physics backward in time to predict events whose effects might be observable in the 2.75 K radiation today. The spectrum and isotropy are being studied with sophisticated microwave radiometers on the ground, in balloons, and in satellites. The results are as predicted by the simple theory: the spectrum is that of a blackbody (to a few percent) and the radiation is isotropic (to 0.01 percent) except for a local effect due to our motion through the radiation."

"David Todd Wilkinson died on 5 September 2002 in Princeton, New Jersey, after having battled cancer for 17 years. His role in the measurements of the thermal (CMB) was key to the completion of the program of cosmological tests that began with Edwin Hubble’s discovery of the expanding universe in 1929."

"… the technical question was, “Is the Big Bang the right story? Is the expanding universe the right story?” And there are a lot of sub-questions in that. If it is the right story, then how did the galaxies come? Where did they come from? Because it seemed quite mysterious. People were just beginning to realize that there was a structure that had been seen in the maps of where the galaxies are located, and that we had no clue what made that happen. So what is there to measure? Well, there’s not very many things to measure. You can measure the galaxies or you can look for this cosmic background radiation. And if you could measure it, it would tell you something that you never knew before."

"NASA’s Cosmic Background Explorer satellite mission, the COBE, laid the foundations for modern cosmology by measuring the spectrum and anisotropy of the and discovering the . ... The COBE observed the universe on the largest scales possible by mapping the cosmic microwave and infrared background radiation fields and determining their spectra. It produced conclusive evidence that the hot Big Bang theory of the early universe is correct, showed that the early universe was very uniform but not perfectly so, and that the total luminosity of post–Big Bang objects is twice as great as previously believed."

"The Cosmic Background Explorer (COBE) satellite, under study by NASA since 1976, will map the spectrum and the angular distribution of diffuse radiation from the universe over the entire wavelength range from 1 micron to 1.3 cm. It carries three instruments: a set of differential s (DMR) at 23.5, 31.4, 53, and 90 , a far infrared absolute (FIRAS) covering 1 to 100 cm-1, and a covering 1 to 300 s. They will use the ideal space environment, a one year lifetime, and standard instrument techniques to achieve orders of magnitude improvements in sensitivity and accuracy, providing a fundamental data base for cosmology. The instruments are united by common purpose as well as similar environmental and orbital requirements. The data from all three experiments will be analyzed together, to distinguish nearby sources of radiation from the cosmologically interesting diffuse background radiations."

"... in the seventies the work of and Rubin ... looked at galaxies and found evidence for dark matter in every single one."

""What is the Universe made of?" The question is one of the deepest unanswered mysteries in all of human existence. Solving the puzzle has been my life's work and is the hottest research topic in cosmology and particle physics today."

"s and massive s in the mass range 2–20 have been proposed as candidates to provide the in the halo of our galaxy."

"... On my father’s side, I come from a family of artists and architects. My grandfather was a Bauhaus architect in Germany and my grandmother was a painter. When my father as a child was tinkering around in the basement with chemistry sets, his mother said to him, “You’ll never make a living in science. Why don’t you go into the family business, the arts?” Well, he defied that advice. He was actually a PhD student with Heisenberg in Göttingen, and then a post-doc with Enrico Fermi in Chicago. After that he went to Caltech where he switched to biology and became one of the founders of the field of molecular biology. He founded the department of molecular biology at the . My mother was from a town on the Swiss-German border called , and she actually got her PhD in biology at age 22."

"Hunter has pointed out that by using a Jupiter swingby to approach the Sun and then giving a velocity boost at perihelion, a solar system escape velocity... is possible with present-day chemical rockets... [M]ost other stars should have planets (or companion stars) with characteristics sufficiently close to... the Jupiter-Sun system to use this launch strategy in reverse to slow down in the other solar system."

"[T]he problem of interstellar travel has been reduced to... transporting a von Neumann machine to another stellar system. This can be done even with present-day rocket technology."

"[T]hus... any intelligent species would develop at least the rocket technology capable of... a travel velocity ves of 3 x 10-4c. At this velocity the travel time to the nearest stars would be between 104 and 105 years. This... would necessitate... self-repair capacity... Nuclear power-souces would supply the power... If power utilization during the free-fall period was... low, even chemical reactions could supply the power."

"Even if there were no planets in the stellar system... the... machine could be programmed to turn some of the material into an O'Neill colony..."

"[T]he von Neumann machine would be programmed to explore the stellar system... and relay information... back to the original solar system..."

"[T]he information to manufacture a human being is contained in the genes of a single human cell. Thus if an extraterrestrial intelligent species possessed the knowledge to synthesize a living cell—and... experts assert... the human race could develop such knowledge within 30 years—they could program a von Neumann machine to synthesize a fertilized egg cell of their species. If they possessed artificial womb technology—and such... is in the beginning stages... on Earth... they could... synthesize members of their species... As suggested by Eiseley... these beings could be raised... by the robots... free to develop their own civilization..."

"As ves is of the same order as the stellar random motion velocities, very sensitive guidance would be required... not... an insuperable problem with the assumed level of computer technology."

"The payload of a probe to another stellar system would be a self-reproducing universal constructor with human level intelligence (...a von Neumann machine) together with an engine... travelling... within the stellar target system—[the engine] could be an electric propulsion system... or a ..."

"What one needs is a self-reproducing universal constructor... a machine capable of making any device... capable of making a copy of itself. Von Neumann has shown... that such... is theoretically possible, and... a human being is a universal constructor specialized to perform on the surface of the Earth."

"[M]aterials [to reproduce the von Neumann machine] should be available in virtually any stellar system—including systems—in the form of meteors, asteroids, comets, and other debris from the formation of the stellar system. ...[M]aterial in asteroids are highly differentiatied; many... are largely nickel-iron, while others contain large amounts of hydrocarbons."

"I shall assume that such a species will... develop a self-replicating universal constructor with intelligence comparable to the human level—such a machine should be developed within a century, according to the experts...—and... combined with present-day rocket technology would make it possible to explore and/or colonize the Galaxy in less than 300 million years, for an initial investment less than the cost of operating a 10 MW microwave beacon for several hundred years, as proposed by SETI..."

"It is a deficiency in computer technology, which prevents us from beginning the exploration of the Galaxy tomorrow."

"As the copies of the space probe were made, they would be launched at the stars nearest the target star. When these probes reached these stars, the process would be repeated... until the probes covered all the stars of the Galaxy."

"[A] von Neumann cannot become obsolete... instructed by radio to make the latest devices..."

"[I]s the God (...Person ...) the God? ...the uncreated Creator of the ...universe ...Who exists necessarily ... i.e., the Person's nonexistence would be a logical contradiction."

"[T]hat any intelligent species which develops... interstellar communication will also have... rocketry... is... a consequence of the principle of mediocrity... (that our own evolution is typical)... [T]he human species developed rockets 600 years before... radio waves..."

"[A]n intelligent species with the technology for interstellar communication would necessarily develop the technology for interstellar travel, and this would automatically lead to the exploration and/or colonization of the Galaxy in less than 300 million years."

"[I]t seems likely that a species engaging in interstellar communication would possess a fairly sophisticated computer technology. ...Sagan has asserted that 'Communication with extraterrestrial intelligence... will require... computer actuated machines with abilities approaching... intelligence'."

"[T]he claim that the central concern of religion is nonsense. Throughout human history, the central concern of religion has been human self-interest. In the Judeo-Christian-Islamic tradition, all morality has been obtained from declarative sentences of the form "Thou shalt not kill—because you'll go to Hell if you do!" In the Hindu-Buddhist tradition... "...—because you'll be born as a cockroach if you do!" ...In both cases, the appeal is to physics, not to fundamental moral postulates."