A mysterious force is accelerating the expansion of the cosmos. The strange effect works against gravity, pushing all matter outward
By José Tadeu Arantes /
This may be the biggest discovery of the late 20th century. And the main scientific challenge of the early new millennium. To the surprise of scientists, two rival teams of astronomers have shown that the expansion of the universe, caused by the Big Bang (the great explosion that gave rise to the cosmos), is not slowing down due to the gravitational pull of matter. On the contrary, it has been accelerating more and more. In other words, a strange force continues to push the universe "outward," causing galaxies to move away from each other at ever-increasing speeds.
Just a year ago, it was thought that the expansion of the cosmos might even be completely halted. And, from there, gravitational attraction would pull everything back, until a new super-explosion occurred - the Big Crunch. This catastrophic scenario is now completely discarded. Driven by the mysterious force that works against gravity, the universe seems destined to expand forever. Until the end of time. The new discovery should cause a revolution in physics. It also leads to a revision of the calculations of the age of the universe. It may be much older than previously thought.
The discovery of the accelerating expansion of the universe was announced simultaneously by two multinational teams of astronomers: the High Z Supernova Search Team, led by Australian Brian Schmidt, and the Supernova Cosmology Project, by American Saul Perlmutter (read box below), both strong candidates for this year's Nobel Prize in Physics. The news is so extraordinary that astronomers around the world are trying to disprove it. If confirmed, however, it should change our entire view of reality. For it will reveal the existence of a force - previously unknown and whose physical nature is a complete mystery - that acts in the opposite direction to gravity, pushing material bodies away from each other. The only explanation found so far for this repulsive force is that it would be produced by the vacuum.
It seems absurd. But, in quantum physics, vacuum is a formidable reservoir of energy, which creates and annihilates particles non-stop. No less than 70% of the universe's energy would be associated with this vacuum, while matter itself would contribute only 30%. "We don't yet have sufficiently sophisticated physics to deal with the vacuum. This means that 70% of the universe's content has never been explored by science," emphasizes astronomer Amâncio Friaça, from the Astronomical and Geophysical Institute (IAG), of USP.
But the astonishment caused by the new discovery doesn't end there. The vacuum's energy content can be up to 10120 times greater than what is needed to cause the universe's acceleration. This fantastic figure corresponds to the digit 1 followed by 120 zeros: a simply unimaginable number. "Investigating the nature of this vacuum will be the great scientific challenge of the coming decades," Friaça enthuses. "We are living in a privileged and exciting moment in the history of science."
The existence or not of the repulsive force has direct consequences on the calculation of the age of the universe. If it exists - that is, if the expansion of the cosmos is indeed accelerating, as the two teams of astronomers claim - then the universe must be older than previously assumed. Because, having had a lower expansion velocity in the past than today, it probably needed more time to reach its current size (see illustration). A decelerating expansion, as was imagined until last year, gives the universe an age of 9.3 billion years. With accelerated expansion, it is possible to extend the cosmos's lifespan to 13.4 or even 15 billion years. This gain of 4.1 to 5.7 billion years is essential to reconcile the age of the universe with that of our galaxy, estimated at about 12 billion years.
How the discovery was made
The researchers discovered that the expansion of the universe was accelerating thanks to the observation of a specific class of stars, type 1a supernovae. The luminous radiation emitted by these celestial bodies is so rigorously uniform that astronomers use them as universal standards. The idea of the Supernova Cosmology Project and High Z Supernova Search teams was then to compare the brightness of nearby supernovae with those very far away. After years of systematic observations, the two teams independently reached the same result: the apparent luminosity of distant stars was lower than expected.
By confronting these data with the predictions of theoretical models, the groups concluded that this was due to an acceleration in the recession velocity of the galaxies where these stars were located. A discovery like this would not have been possible a decade ago. The Supernova Cosmology Project and the High Z Supernova Search had at their disposal the most sophisticated set of equipment ever used in astronomy: the Hubble Space Telescope, the fantastic 10-meter Keck telescopes located in Hawaii, and several 4-meter telescopes spread around the world.
Using the 4-meter telescopes, astronomers photographed several regions of the sky and photographed them again three weeks later. Each image obtained contained records of hundreds of millions of galaxies. The next step was to track the differences between the two images of each region - something more difficult than finding a needle in a haystack. But not impossible when you have the help of cutting-edge computers. These differences - reasoned the astronomers - could only be the result of supernova explosions that occurred during the time interval between one photo and the next. Among the supernovae found, those of type 1a were selected. About a hundred celestial bodies were found, some very old, up to 10 billion years old. Once the desired objects were located, the teams then used Hubble and Keck to determine, with the greatest possible accuracy, their distance and apparent luminosity. And, by comparing the two sets of numbers, they arrived at the acceleration of the cosmos's expansion.
The Age of the Cosmos
Last May, working with data collected by the Hubble Space Telescope, a team of astronomers led by American Wendy Freedman announced they had arrived at a new value for the age of the universe: 12 billion years. This estimate was based on an accurate calculation of the so-called Hubble constant, which relates the speed at which galaxies move away from each other and their relative distance.
Previously, the calculation of the constant was so imprecise that astronomers used to differ by a factor of two - meaning they attributed an age to the universe ranging from 10 to 20 billion years. This imprecision has now been reduced to only 10%. Freedman achieved this by studying about 800 Cepheid variable stars - stars with variable brightness that astronomers use to measure distances. Using Cepheids as markers, he determined the distance from Earth to 18 galaxies located within a radius of 65 million light-years. By combining these data with the velocities of these same galaxies, estimated from the characteristics of the light we receive from them, astronomers arrived at the new value of the Hubble constant and, from there, to 12 billion years. This is the best determination of the age of the universe ever made. But it is not yet definitive. Especially because it is based only on the current value of the cosmos's expansion rate. If this rate is accelerating, the age should exceed the 13.4 billion year mark.
The Incredible Inflation
Expansion leads to cooling. And it was this cooling that enabled the structuring of matter, imposing dramatic transformations on the universe. Moments after the Big Bang, the sharp drop in temperature caused an imbalance that violently accelerated the expansion of space. This phenomenon, named inflation, lasted a tiny fraction of a second. But it was so intense that it made the universe go from the volume of a proton to the size that can be observed today by the most powerful telescopes.
Although it had cooled significantly, the temperature was still extremely high - on the order of 1027 Kelvin, that is, a billion billion billion degrees above absolute zero. The universe then consisted of pure energy, in frantic activity, with particles and antiparticles being produced and annihilated non-stop. These first corpuscles belonged to the classes of quarks and antiquarks and leptons and antileptons. One millionth of a second after the Big Bang, the temperature had dropped to the level of 10 trillion degrees, allowing triplets of quarks and antiquarks to bind together to form protons, neutrons, and their respective antiparticles.
Three minutes later, the universe had cooled so much that protons and neutrons could already combine, forming the atomic nuclei of the simplest elements: hydrogen (one proton), deuterium (one proton and one neutron), helium (two protons and two neutrons), lithium (three protons and four neutrons). The cosmic thermometer then registered a mark of one million degrees. This formation of nuclei, known as "primordial nucleosynthesis," continued for about 20 minutes. But it was necessary to wait for 300,000 to 400,000 years of cooling for the nuclei to capture free electrons, forming the first atoms.
And Life Emerges
Before the formation of atoms, the universe was opaque, as the interaction between free electrons and photons (light particles) prevented the propagation of luminous radiation. With the capture of free electrons, photons could travel freely. And the cosmos became transparent. Less than a billion years later, the gravitational attraction between atoms gave rise to the first generations of stars and galaxies. Nucleosynthesis continued inside the stars, enriching the cosmos with atomic nuclei of more complex elements (carbon, oxygen, iron, etc.). These would participate in the formation of later generations of stars - eventually with planets, where, under special circumstances, the organization of matter would create conditions for the emergence of life.
Just One Among Many Universes
What came before the Big Bang? Until a few years ago, this question seemed to make no sense at all. Scientists believed that space and time were born with the Big Bang. Therefore, the word "before" could only be used after this event. Today, people are starting to think differently. "Many cosmologists believe that our cosmos is just one among many universes, perhaps infinite," says astronomer Laerte Sodré, from IAG-USP. "The Big Bang would mark only the birth of our universe, but there would be others, possibly much older."
These universes could have very different physical properties from ours. And, at the current stage of scientific development, they are completely beyond our observational capabilities. "But it's important to be clear that these ideas are purely speculative," warns Sodré. "We do not yet have sufficiently developed physics to understand what happened in the first fractions of a second after the Big Bang, let alone what came before." One of cosmology's virtues, incidentally, is to give humans a small notion of the extent of their ignorance.
As already mentioned, 70% of the universe's energy may originate from a vacuum that science has barely begun to investigate. But that's not all: of the remaining 30% of content, only one-third corresponds to conventional matter, studied by physics. The other two-thirds would be formed by so-called "dark matter," whose nature is completely unknown (read box below). Far from making them depressed, this glimpse of the unknown is a formidable stimulus for creative scientists. "Incredibly interesting things are beginning to emerge for scientific investigation," announces Amâncio Friaça. "Science is just beginning."
Exotic Dark Matter
Proton, electron, neutron: all these concepts associated with conventional matter do not apply to dark matter. Scientists still have no idea of its nature. But they are increasingly convinced that it exists. "This conviction began to solidify in the 1930s," reports Laerte Sodré. "At that time, by studying the behavior of galaxy clusters, the Bulgarian astronomer Fritz Zwicky (1898-1974) calculated the mass that galaxies should have for their relative motion to counterbalance gravitational attraction, preventing them from crushing each other. And he concluded that they contained much more matter than could be attributed to the sum of their stars."
Since then, evidence for dark matter has grown continuously. It would fill the cosmos, forming halos around galaxies that are much larger than their visible core. Although it cannot be detected by conventional means, dark matter would also be present on Earth - even inside human bodies. And its existence could explain many mysteries that have so far defied science.
Universe Expansion
A mysterious force is accelerating the expansion of the cosmos. The strange effect works against gravity, pushing all matter outward
By José Tadeu Arantes /
This may be the biggest discovery of the late 20th century. And the main scientific challenge of the early new millennium. To the surprise of scientists, two rival teams of astronomers have shown that the expansion of the universe, caused by the Big Bang (the great explosion that gave rise to the cosmos), is not slowing down due to the gravitational pull of matter. On the contrary, it has been accelerating more and more. In other words, a strange force continues to push the universe "outward," causing galaxies to move away from each other at ever-increasing speeds.
Just a year ago, it was thought that the expansion of the cosmos might even be completely halted. And, from there, gravitational attraction would pull everything back, until a new super-explosion occurred - the Big Crunch. This catastrophic scenario is now completely discarded. Driven by the mysterious force that works against gravity, the universe seems destined to expand forever. Until the end of time. The new discovery should cause a revolution in physics. It also leads to a revision of the calculations of the age of the universe. It may be much older than previously thought.
The discovery of the accelerating expansion of the universe was announced simultaneously by two multinational teams of astronomers: the High Z Supernova Search Team, led by Australian Brian Schmidt, and the Supernova Cosmology Project, by American Saul Perlmutter (read box below), both strong candidates for this year's Nobel Prize in Physics. The news is so extraordinary that astronomers around the world are trying to disprove it. If confirmed, however, it should change our entire view of reality. For it will reveal the existence of a force - previously unknown and whose physical nature is a complete mystery - that acts in the opposite direction to gravity, pushing material bodies away from each other. The only explanation found so far for this repulsive force is that it would be produced by the vacuum.
It seems absurd. But, in quantum physics, vacuum is a formidable reservoir of energy, which creates and annihilates particles non-stop. No less than 70% of the universe's energy would be associated with this vacuum, while matter itself would contribute only 30%. "We don't yet have sufficiently sophisticated physics to deal with the vacuum. This means that 70% of the universe's content has never been explored by science," emphasizes astronomer Amâncio Friaça, from the Astronomical and Geophysical Institute (IAG), of USP.
But the astonishment caused by the new discovery doesn't end there. The vacuum's energy content can be up to 10120 times greater than what is needed to cause the universe's acceleration. This fantastic figure corresponds to the digit 1 followed by 120 zeros: a simply unimaginable number. "Investigating the nature of this vacuum will be the great scientific challenge of the coming decades," Friaça enthuses. "We are living in a privileged and exciting moment in the history of science."
The existence or not of the repulsive force has direct consequences on the calculation of the age of the universe. If it exists - that is, if the expansion of the cosmos is indeed accelerating, as the two teams of astronomers claim - then the universe must be older than previously assumed. Because, having had a lower expansion velocity in the past than today, it probably needed more time to reach its current size (see illustration). A decelerating expansion, as was imagined until last year, gives the universe an age of 9.3 billion years. With accelerated expansion, it is possible to extend the cosmos's lifespan to 13.4 or even 15 billion years. This gain of 4.1 to 5.7 billion years is essential to reconcile the age of the universe with that of our galaxy, estimated at about 12 billion years.
How the discovery was made
The researchers discovered that the expansion of the universe was accelerating thanks to the observation of a specific class of stars, type 1a supernovae. The luminous radiation emitted by these celestial bodies is so rigorously uniform that astronomers use them as universal standards. The idea of the Supernova Cosmology Project and High Z Supernova Search teams was then to compare the brightness of nearby supernovae with those very far away. After years of systematic observations, the two teams independently reached the same result: the apparent luminosity of distant stars was lower than expected.
By confronting these data with the predictions of theoretical models, the groups concluded that this was due to an acceleration in the recession velocity of the galaxies where these stars were located. A discovery like this would not have been possible a decade ago. The Supernova Cosmology Project and the High Z Supernova Search had at their disposal the most sophisticated set of equipment ever used in astronomy: the Hubble Space Telescope, the fantastic 10-meter Keck telescopes located in Hawaii, and several 4-meter telescopes spread around the world.
Using the 4-meter telescopes, astronomers photographed several regions of the sky and photographed them again three weeks later. Each image obtained contained records of hundreds of millions of galaxies. The next step was to track the differences between the two images of each region - something more difficult than finding a needle in a haystack. But not impossible when you have the help of cutting-edge computers. These differences - reasoned the astronomers - could only be the result of supernova explosions that occurred during the time interval between one photo and the next. Among the supernovae found, those of type 1a were selected. About a hundred celestial bodies were found, some very old, up to 10 billion years old. Once the desired objects were located, the teams then used Hubble and Keck to determine, with the greatest possible accuracy, their distance and apparent luminosity. And, by comparing the two sets of numbers, they arrived at the acceleration of the cosmos's expansion.
The Age of the Cosmos
Last May, working with data collected by the Hubble Space Telescope, a team of astronomers led by American Wendy Freedman announced they had arrived at a new value for the age of the universe: 12 billion years. This estimate was based on an accurate calculation of the so-called Hubble constant, which relates the speed at which galaxies move away from each other and their relative distance.
Previously, the calculation of the constant was so imprecise that astronomers used to differ by a factor of two - meaning they attributed an age to the universe ranging from 10 to 20 billion years. This imprecision has now been reduced to only 10%. Freedman achieved this by studying about 800 Cepheid variable stars - stars with variable brightness that astronomers use to measure distances. Using Cepheids as markers, he determined the distance from Earth to 18 galaxies located within a radius of 65 million light-years. By combining these data with the velocities of these same galaxies, estimated from the characteristics of the light we receive from them, astronomers arrived at the new value of the Hubble constant and, from there, to 12 billion years. This is the best determination of the age of the universe ever made. But it is not yet definitive. Especially because it is based only on the current value of the cosmos's expansion rate. If this rate is accelerating, the age should exceed the 13.4 billion year mark.
The Incredible Inflation
Expansion leads to cooling. And it was this cooling that enabled the structuring of matter, imposing dramatic transformations on the universe. Moments after the Big Bang, the sharp drop in temperature caused an imbalance that violently accelerated the expansion of space. This phenomenon, named inflation, lasted a tiny fraction of a second. But it was so intense that it made the universe go from the volume of a proton to the size that can be observed today by the most powerful telescopes.
Although it had cooled significantly, the temperature was still extremely high - on the order of 1027 Kelvin, that is, a billion billion billion degrees above absolute zero. The universe then consisted of pure energy, in frantic activity, with particles and antiparticles being produced and annihilated non-stop. These first corpuscles belonged to the classes of quarks and antiquarks and leptons and antileptons. One millionth of a second after the Big Bang, the temperature had dropped to the level of 10 trillion degrees, allowing triplets of quarks and antiquarks to bind together to form protons, neutrons, and their respective antiparticles.
Three minutes later, the universe had cooled so much that protons and neutrons could already combine, forming the atomic nuclei of the simplest elements: hydrogen (one proton), deuterium (one proton and one neutron), helium (two protons and two neutrons), lithium (three protons and four neutrons). The cosmic thermometer then registered a mark of one million degrees. This formation of nuclei, known as "primordial nucleosynthesis," continued for about 20 minutes. But it was necessary to wait for 300,000 to 400,000 years of cooling for the nuclei to capture free electrons, forming the first atoms.
And Life Emerges
Before the formation of atoms, the universe was opaque, as the interaction between free electrons and photons (light particles) prevented the propagation of luminous radiation. With the capture of free electrons, photons could travel freely. And the cosmos became transparent. Less than a billion years later, the gravitational attraction between atoms gave rise to the first generations of stars and galaxies. Nucleosynthesis continued inside the stars, enriching the cosmos with atomic nuclei of more complex elements (carbon, oxygen, iron, etc.). These would participate in the formation of later generations of stars - eventually with planets, where, under special circumstances, the organization of matter would create conditions for the emergence of life.
Just One Among Many Universes
What came before the Big Bang? Until a few years ago, this question seemed to make no sense at all. Scientists believed that space and time were born with the Big Bang. Therefore, the word "before" could only be used after this event. Today, people are starting to think differently. "Many cosmologists believe that our cosmos is just one among many universes, perhaps infinite," says astronomer Laerte Sodré, from IAG-USP. "The Big Bang would mark only the birth of our universe, but there would be others, possibly much older."
These universes could have very different physical properties from ours. And, at the current stage of scientific development, they are completely beyond our observational capabilities. "But it's important to be clear that these ideas are purely speculative," warns Sodré. "We do not yet have sufficiently developed physics to understand what happened in the first fractions of a second after the Big Bang, let alone what came before." One of cosmology's virtues, incidentally, is to give humans a small notion of the extent of their ignorance.
As already mentioned, 70% of the universe's energy may originate from a vacuum that science has barely begun to investigate. But that's not all: of the remaining 30% of content, only one-third corresponds to conventional matter, studied by physics. The other two-thirds would be formed by so-called "dark matter," whose nature is completely unknown (read box below). Far from making them depressed, this glimpse of the unknown is a formidable stimulus for creative scientists. "Incredibly interesting things are beginning to emerge for scientific investigation," announces Amâncio Friaça. "Science is just beginning."
Exotic Dark Matter
Proton, electron, neutron: all these concepts associated with conventional matter do not apply to dark matter. Scientists still have no idea of its nature. But they are increasingly convinced that it exists. "This conviction began to solidify in the 1930s," reports Laerte Sodré. "At that time, by studying the behavior of galaxy clusters, the Bulgarian astronomer Fritz Zwicky (1898-1974) calculated the mass that galaxies should have for their relative motion to counterbalance gravitational attraction, preventing them from crushing each other. And he concluded that they contained much more matter than could be attributed to the sum of their stars."
Since then, evidence for dark matter has grown continuously. It would fill the cosmos, forming halos around galaxies that are much larger than their visible core. Although it cannot be detected by conventional means, dark matter would also be present on Earth - even inside human bodies. And its existence could explain many mysteries that have so far defied science.
