Tag Archives: standard cosmological model

The Cosmological Constant. Einstein's 'greatest blunder' is an expanding problem

The Cosmological Constant: How Einstein’s ‘greatest blunder’ became an expanding problem

The Cosmological Constant. Einstein's 'greatest blunder' is an expanding problem
The Cosmological Constant–represented by the Greek letter lambda–Einstein’s ‘greatest blunder’ is an expanding problem

The cosmological constant is a problem. Actually, that is an understatement, the cosmological constant was a problem, is a problem, and may always be a problem. To understand why this little constant has caused so stress for cosmologists, it is necessary to divide its history into two very distinct eras, and perhaps then, consider its future. Thus this introduction is to the cosmological constant what Marley’s ghost was to Scrooge. A warning of a trip through its history, its future, and perhaps, offering it a hint of redemption. 

The current theoretical estimations of the cosmological constant differ from the experimental measurements by such a shocking and huge magnitude, that it has often been referred to as ‘the worst prediction in the history of science.’ And as these values are provided by the non-unitable fields of physics quantum field theory and general relativity, respectively, finding an agreeable value for the constant, or even the reason why the values diverge so greatly could be the key to finding a quantum theory of gravity. 

Before embarking on that trip, let’s first meet our Scrooge — the central character of this potential redemption arc. The cosmological constant represents now, something different than it did when it was first introduced.

The easiest way to understand what it means now is by considering dark energy — the hypothetical force driving the Universe apart — to be the physical manifestation of the cosmological constant. As such, again solving the mystery of the cosmological constant could be the key to discovering exactly what dark energy is, and, in turn, discovering what the final fate of the Universe will look like. 

In many ways, the cosmological constant can be considered as a ‘counterpoint’ to gravity, a value for a force that repels as gravity attracts. This is something that links its present with its past. 

Einstein’s biggest blunder?

It’s sometimes mind-boggling to consider that the biggest problem in modern physics is a hangover from 1917. With all the advancements we have made in terms of understanding our Universe, how can this one little element, provide such a challenge?

The key to understanding why the cosmological constant has been such a thorn in the side of physics is understanding how it confounded the greatest physicist who ever lived — Albert Einstein. 

The cosmological constant, often represented by the Greek letter Lambda (λ), was added to Einstein’s field equations to balance the force of gravity. This is explainable by the fact that if only gravity acts in the Universe — an attractive force — then how can it not be shrinking? How did it form at all, if all matter is naturally drawn together?

Einstein felt that his field equations needed a repulsive factor to counter-balance the attractive force of gravity, and if this sounds like an ad-hoc solution — a fudge factor — that is because it was. Not only was the cosmological constant something of an arbitrary ‘fix’ it also made the field equations unstable. A slight variation should, according to these revised equations, cause the Universe to fall out of its static state. For example, if separation increases, gravitational attraction decreases and repulsion increases — resulting in further deviation from the initial state.  

Einstein was influenced to introduce the cosmological constant by the fact that the scientific consensus in 1917 was that the Universe was static — neither expanding nor contracting — and Einstein agreed with the consensus. Unfortunately, his field equations disagreed.

The field equations of general relativity did not allow for a static universe, predicting that the Universe should either be contracting or expanding. Thus, the first role that the cosmological constant was to provide negative pressure to counterbalance gravity. The argument underpinning this addition was that even empty space-time has a gravitational influence, a so-called vacuum energy. 

For twelve years, the cosmological constant remained in the field equations, fulfilling this role. But, trouble was brewing on the horizon, and by ‘the horizon,’ we mean the most distant horizon imaginable — the very edge of the Universe. 

Our understanding of the cosmos was about to change forever…

Hubble trouble — an expanding problem

It may be slightly difficult to believe today, but just 90 years ago we understood far less about the cosmos and the Universe around us. The idea of billions of galaxies outside of our Milky Way was almost undreamt of, as was the idea that these galaxies could be receding from each other as space expands. Likewise, the idea that the Universe could have inflated from an infinitely small point — the concept of the ‘Big Bang’ was pure fantasy. 

In 1929, Edwin Hubble’s seminal paper “A relation between distance and radial velocity among extra-galactic nebulae” would change this thinking forever. Hubble showed that the Universe was not infinite in either its reach or its age. In this relatively short paper, the astronomer presented the first observational evidence that distant galaxies are moving away from us, and further to this, the more distant they are, the more rapidly they recede. 

A simple sketch of a graph showing velocity against distance for distant galaxies revealed a profound insight about the Universe around us (Hubble, 1929) 

What Hubble was unaware about when he published his 1929 paper, was that other physicists had already provided solutions to Einstein’s field equations that his results confirmed. Both Alexander Friedmann, a Russian cosmologist, and Georges Lemaitre, a Catholic priest, mathematician, astronomer, and professor of physics, had provided solutions to the field equations that showed an expanding Universe. Even with this theoretical basis, Einstein wanted to see this evidence for himself, not being quite ready to scrap his cosmological constant and accept a non-static universe.

I need to see this for myself. Einstein visits the Mount Wilson observatory and meets Hubble. (CalTech)

On January 29th 1931, Edwin Hubble met Einstein at Mount Wilson, taking him to see the famous 100-inch telescope where the astronomer had made the observation that doomed the first iteration of the cosmological constant. Shortly after Einstein published his first paper with revised field equations omitting lambda. He deemed the constant ‘redundant’ as relativity could explain the expansion of the Universe without it.

George Gamow, physicist and cosmologist, remarked in a 1956 Scientific American article and then later in his autobiography, that Einstein had confided in him that the introduction of the cosmological constant was his ‘biggest blunder.’ The remark has now passed into the lore surrounding the great scientist, and even though we can’t be certain that he actually said it, he very likely believed it. 

Yet, despite Einstein’s dismissal of the cosmological constant, many physicists were not quite ready to give up on this element of general relativity just yet. They argued without a cosmological constant term, models of the evolution of the cosmos would predict a universe with an age younger than the oldest stars within it. 

And though Einstein was unmoved by this argument, in 1998, 43 years after his death, the cosmological constant and the symbol that represents it would be rescued from obscurity and drafted to explain a new, but related conundrum.

The modern cosmological constant and dark energy

There is something of a pleasing irony about the fact that it was the discovery of the Universe expanding that consigned the cosmological constant to the dustbin and it was the equally important discovery that this same expansion is accelerating that saved it. 

During the cosmological constant’s ‘downtime’ our understanding of the origins of the Universe underwent a revolution. Cosmologists were able to deduce that regions of the Universe now separated by unimaginable distances were once in close proximity. The idea that the Universe expanded in a period of rapid inflation from an infinitely small point — though this point would become progressively smaller — to the vast entity we see today was accepted and termed the ‘Big Bang.’ 

Yet the commonly held idea that this period of rapid inflation had given way to a more steady rate was challenged in 1998.

Three distant Type Ia supernovas, as observed by the Hubble Space Telescope in 1997. Since Type Ia supernovas have the same luminosity, they are used in measuring dark energy and its effects on the expansion of the universe. The bottom images are details of the upper wide views. The supernovas at left and centre occurred about five billion years ago; the right, seven billion years ago. (Photo AURA/STScI/NASA/JPL (NASA photo # STScI-PRC98–02a-js))

In the mid-nineties, cosmologists had used solutions to the field equations of general relativity to assess the geometry of the Universe, determining that it is flat. This left some problems to be addressed. In a flat universe, we should have a matter/energy density which matches a value known as the critical density. Yet all the matter and energy that we can observe accounts for only a third of this value. Further to this missing energy problem, the flat universe suffers from a cosmic age problem, why do the oldest stars appear older than the predicted age of a flat universe?

The expansion of the Universe as depicted in this image may have forced the cosmological constant into a hiatus, but it was out for good. C. FAUCHER-GIGUÈRE, A. LIDZ, AND L. HERNQUIST, SCIENCE 319, 5859 (47)

One solution to these problems could arise if the Universe is filled with a fluid of negative pressure, a ‘dark energy’ that accounts for the energy deficit and provides accelerated expansion that would neatly explain the Universe taking a longer time to reach its current state. To measure this changing rate of expansion researchers would need a tool that could measure extraordinarily large cosmic distances — as large as 5 billion light-years, in fact. 

In 1998, astronomers found evidence of such a theoretical fluid from observations of the redshifts of distant yet incredibly, bright Type Ia supernovae — often referred to as ‘standard candles’ due to their reliability in measuring cosmic distances. And of course, scientists would need a symbol to represent dark energy within their equations. As cosmology already had such a representation of negative pressure, why not simply resurrect it and place it back in the equations of general relativity?

But, they should not have been surprised, given its history, that re-employing the cosmological constant would bring new problems.

Still crazy after all these years…

The new issues with the cosmological constant very much reflect the major hurdles within physics as it currently stands. Whilst revolutions were being made on incredibly large scales thanks to cosmology, our understanding of the incredibly small was burgeoning thanks to the success of quantum physics. 

The problem arises from the fact that quantum physics — and in particular quantum field theory — and general relativity can not be reconciled, there is no theory of quantum gravity. 

If we are creative and slightly whimsical, we could perhaps give this struggle to unify these disciplines a value — 10¹²¹ — the magnitude between which the quantum field theory’s theoretical prediction of the cosmological constant and the observed value provided by cosmology. This massive disparity–often described as ‘the worst theoretical prediction in the history of science’– arises from the fact that quantum field theory predicts that virtual particles are popping in and out of existence at all times — an idea that may sound ridiculous but has been experimentally verified — even in the vacuum of space. Thus particles should have a measurable effect on the vacuum energy driving the expansion of the Universe but such an effect isn’t measured by cosmologists observing the redshifts of Type Ia supernovae.

Virtually impossible: A pair of virtual particles, one with a positive charge and one with a negative charge briefly come into existence and disappear. Such particles should have an effect on the expansion of the Universe according to quantum field theory.

There are, of course, solutions. Dark energy could be associated with some, as yet undiscovered field, which fills space in a way similar to that of the Higgs field from which the recently discovered Higgs boson emerges. Or perhaps other constants that occupy an unchallenged place in our equations of gravity aren’t constants at all but vary with time, as University of Geneva cosmologists Lucas Lombriser suggests. More extreme solutions lie in the suggestion that Einstein’s theory of gravity must be modified to account for dark energy — all though this family of theories, MONDS, are steadily moving out of favour within the physics community.

Whatever the solution to this problem is, it has a remarkable impact on the future of the Universe. Determining the true value of the cosmological constant and the strength of dark energy driving this accelerating expansion will ultimately tell us if the Universe’s final fate is to rip apart or violently crush together. 

Whether by ‘Big Rip’ or ‘Big Crunch’ the Universe’s end will be determined by the value of the cosmological constant. A value that still continues to evade us and confuse us as much as it did Einstein.

Sources

Ta-Pei Cheng, Relativity, Gravitation and Cosmology, Oxford Press, (2010).

Robert Lambourne, Stephen Serjeant, Mark Jones, An Introduction to Galaxies and Cosmology, Cambridge Press, (2015).

Frank Close, The New Cosmic Onion, Taylor &Francis, (2007).

Cormac O’Raifeartaigh, Investigating the legend of Einstein’s “biggest blunder”, Physics Today, (2018)

Matts Roos, Introduction to Cosmology, Wiley, (2003). 

Ancient galaxies from the study are visible to ALMA (right) but not to Hubble (left). Credit: © 2019 Wang et al.

‘Hidden’ ancient galaxies discovery may redefine our understanding of the Universe

The discovery of 39 ‘hidden’ ancient galaxies urges scientists to rethink their theories of fundamental aspects of the Universe — including supermassive black holes, star formation rates, and the ever-elusive, dark matter.

Ancient galaxies from the study are visible to ALMA (right) but not to Hubble (left). Credit: © 2019 Wang et al.

In an unprecedented discovery of astronomers, researchers have utilised the combined power of a multitude of observatories across the globe to discover a vast array of 39 previously hidden galaxies.

The finding — described by the researchers from the University of Tokyo as a ‘treasure trove’ — is the first multiple discoveries of this kind. But the finding is significant for more than its size alone.

In addition to containing a wealth of newly discovered ancient galaxies, an abundance of this particular type of galaxy suggests that scientists may have to refine current models of the universe.

This is because our current understanding of the universe and how it formed is built upon observations of galaxies in ultraviolet light. But observations in these wavelengths under-represent the most massive galaxies — those with high dust content and crucially, the most ancient.

This means that a discovery of such galaxies — such as the one just made — must force us to reconsider the rates of star formation in the early universe. The study explains that the population of stars discovered may mean that star formation rates were actually ten times greater in early epochs than previous estimates held.

There are also particular ramifications for our understanding of both supermassive black holes and their distribution, and for the concept of dark matter — the elusive substance which makes up 80% of the matter in the universe.

Despite the wealth of astronomical data that has become available to scientists since the launch of the Hubble Space Telescope, researchers at the Institute of Astronomy in Toyko were aware there were things that Hubble simply couldn’t show us. It was these things — fundamental pieces of the cosmic puzzle — that they wanted to investigate.

They achieved this by unifying different observatories, using them to look more deeply in the Universe than Hubble alone could do. This is what led them to this huge collection of galaxies.

Researcher Tao Wang describes the uniqueness and magnitude of the team’s discovery: “This is the first time that such a large population of massive galaxies was confirmed during the first two billion years of the 13.7-billion-year life of the universe.

“These were previously invisible to us.”

Wang continues: “This finding contravenes current models for that period of cosmic evolution and will help to add some details, which have been missing until now.”

A different view of the universe

Wang explains that if we could see these galaxies and the light they shed, our view from the Milky way would be significantly different: “For one thing, the night sky would appear far more majestic. The greater density of stars means there would be many more stars close by appearing larger and brighter.

“But conversely, the large amount of dust means farther-away stars would be far less visible, so the background to these bright close stars might be a vast dark void.”

The galaxies have been difficult to see from Earth due to how faint they are. Were we able to see these stars, their density would make the night sky majestic, Wang says.

The light from these galaxies also has to battle extinction — the absorption of light) by intervening interstellar dust clouds. The light from the galaxies also has to travel great distances meaning the wavelength is redshifted by the expansion of the universe making it even less visible.

Professor Kotaro Kohno. Credit: © 2019 Rohan Mehra — Division of Strategic Public Relations

Professor Kotaro Kohno explains that this phenomenon is how the galaxies escaped Hubble’s gaze: “The light from these galaxies is very faint with long wavelengths invisible to our eyes and undetectable by Hubble.

“So we turned to the Atacama Large Millimeter/submillimeter Array (ALMA), which is ideal for viewing these kinds of things. I have a long history with that facility and so knew it would deliver good results.”

This redshift due to cosmic expansion does have its advantages, however. It allows astronomers to estimate not just the distances to the galaxies in question, but it also allows them to calculate just how long ago the light was emitted.

The hidden implications of these hidden galaxies

The team’s finding is so controversial and poses such a radical rethink that they found their fellow astronomers were initially reluctant to believe they had found what they claimed.

A few of the 66 radio telescope antennas that make up ALMA. Credit: © 2019 Kohno et al.

Wang explains: “It was tough to convince our peers these galaxies were as old as we suspected them to be. Our initial suspicions about their existence came from the Spitzer Space Telescope’s infrared data.

“But ALMA has sharp eyes and revealed details at submillimeter wavelengths, the best wavelength to peer through dust present in the early universe. Even so, it took further data from the imaginatively named Very Large Telescope in Chile to really prove we were seeing ancient massive galaxies where none had been seen before.”

The discovery has the potential to reshape our ideas of the supermassive black holes that scientists currently believe nestle at the centre of most galaxies.

Kohno elaborates: “The more massive a galaxy, the more massive the supermassive black hole at its heart.

“So the study of these galaxies and their evolution will tell us more about the evolution of supermassive black holes, too.”

Kohno also explains that some ideas regarding dark matter may have to be revised, too: “Massive galaxies are also intimately connected with the distribution of invisible dark matter. This plays a role in shaping the structure and distribution of galaxies. Theoretical researchers will need to update their theories now.”

In addition to the potential shake up the team believes that their findings may already present, they expect more surprises to come.

Wang concludes: These gargantuan galaxies are invisible in optical wavelengths so it’s extremely hard to do spectroscopy, a way to investigate stellar populations and chemical composition of galaxies. ALMA is not good at this and we need something more.

“I’m eager for upcoming observatories like the space-based James Webb Space Telescope to show us what these primordial beasts are really made of.”


Original research: T. Wang, C. Schreiber, D. Elbaz, Y. Yoshimura, K. Kohno, X. Shu, Y. Yamaguchi, M. Pannella, M. Franco, J. Huang, C.F. Lim & W.H. Wang. A dominant population of optically invisible massive galaxies in the early Universe. Nature. DOI: 10.1038/s41586–019–1452–4

The galaxy Centaurus A, seen here centered. Credit: Christian Wolf & SkyMapper Team/Australian National University.

Dwarf satellite galaxies are challenging the standard cosmological model

Astronomers are puzzled by a handful of dwarf galaxies that revolve around a larger elliptical galaxy called Centaurus A in a peculiar fashion.

The galaxy Centaurus A, seen here centered. Credit: Christian Wolf & SkyMapper Team/Australian National University.

The galaxy Centaurus A, seen here centered. Credit: Christian Wolf & SkyMapper Team/Australian National University.

The galaxies’ unusual behavior doesn’t ‘fit’ within any framework described by the standard cosmological model, also known as the Lambda Cold Dark Matter cosmological model of the Universe or simply as the Lambda CDM (λCDM).

This model is our best interpretation so far of the origin, evolution, and behavior of the known universe around us, from the Big Bang to present day. The motion of Centaurus A’s galactic satellites might leave scientists scrambling for a new model that fits with the observations.

Challenging the established physics

Less massive objects are always attracted by more massive ones — such is the nature of the universe, as spurred by the force of gravity. The moon gravitates around Earth, which revolves around the sun, which in turn revolves around the Milky Way’s center of mass, corresponding to a supermassive black hole. Also revolving around the Milky Way are other, smaller galaxies called satellite galaxies. Scientists have always found it odd that our galaxy, the spiral-armed Milky Way, has several satellites that orbit in a flat plane, just like planets behave in a solar system, rather than exhibiting a random, spherical distribution as the λCDM predicts. The same has been observed in the nearby Andromeda galaxy whose satellites are also arrayed in a flat plane.

Previously, scientists have always shrugged off these surprising patterns as statistical anomalies. But the latest detailed observations of Centaurus A and its satellites add another exception to the rule. According to an international team of researchers, 14 out of the 16 satellite galaxies orbit their larger host galaxy together in the same plane. With so much evidence piling up, it’s hard to ignore the observation and discard it as a mere glitch in the matrix.

There are some ideas that might explain what the scientists are seeing. Just like the moon’s gravity leads to tides here on Earth, so can the gravity of a larger galaxy rip out stars and gas from another galaxy. Due to this interaction, it may be possible that some dwarf galaxies orbit the larger ones in a flat planet and along the same direction.

It’s not at all clear at this point, however, how stable tidal dwarf galaxies are. Without this information, it’s difficult to come to a reasonable interpretation, especially considering that the λCDM hinges on the assumption that dwarf galaxies are the building blocks of the known universe. These were the very first galaxies ever, which then merged to form larger ones, such as the Milky Way.

Another hypothesis is that, as the λCDM model implies, dark matter filaments that shape the arrangement of galaxies can be as long as 10 to 20 times the distance from Earth to Centaurus A. That’s mind-boggling scale of dark matter distribution might be what’s causing this planar structure like the one scientists are observing now.

According to model simulations with dark matter, however, only half a percent of satellite systems in the local universe at most should behave this way. But what astronomers are seeing can’t be any coincidence.

“Coherent movement seems to be a universal phenomenon that demands new explanations,” says Oliver Müller, an astronomer at the University of Basel’s Department of Physics and co-author of the new study published in Science.

While the new data is contradicting the λCDM, that doesn’t mean that the model is done for. So much about everything else scientists are tracking in the universe works within the boundary of the standard cosmological model that it would be quite foolish to discard it at the first bumps in the road. It will take far more contradictory evidence before scientists start looking for an alternative. The upcoming James Webb Space Telescope will help astronomers study more galaxies and their satellites, data that will help scientists draw better conclusions.

“At worst, we improve our understanding of galaxy formation; at best, we are led to a deeper understanding of the laws of physics,”  said astrophysicist Mike Boylan-Kolchin of the University of Texas, who commented on the study for Ars Technica. “The distribution of satellites around nearby galaxies does not yet rise to the level of a crisis driving us to a new understanding, but the results of Müller et al. raise the stakes.”