What is the Steady State Hypothesis?

What is the Steady State Hypothesis?

https://ift.tt/38dQpwH

When it comes to our cosmic origins, a number of theories have been advanced throughout the course of history. Literally every culture that’s ever existed has had its own mythological tradition, which naturally included a creation story. With the birth of the scientific tradition, scientists began to understand the Universe in terms of physical laws that could be tested and proven.

With the dawn of the Space Age, scientists began testing cosmological theories in terms of observable phenomena. From all of this, a number of theories emerged by the latter half of the 20th century that attempted to explain how all matter and the physical laws governing it came to be. Of these, the Big Bang Theory remains the most widely accepted while the Steady-State Hypothesis has historically been its greatest challenger.

The Steady-State model states that the density of matter in the expanding universe remains unchanged over time because of the continuous creation of matter. In other words, the observable Universe essentially remains the same regardless of time or place. This places it in sharp contrast to the theory that the majority of matter was created in a single event (the Big Bang) and has been expanding ever since.

Illustration showing three steps astronomers used to measure the universe’s expansion rate (Hubble constant). Credits: NASA, ESA, A. Feild (STScI), and A. Riess (STScI/JHU)

Origins

While the notion of a stable and unchanging Universe has been embraced throughout history, it was not until the early modern period that scientists began to interpret this in astrophysical terms. The first clear example of this being argued in the context of astronomy and cosmology was in Isaac Newton‘s Mathematical Principles of Natural Philosophy (Philosophiæ Naturalis Principia Mathematica) published in 1687.

In Newton’s magnum opus, he conceptualized the Universe beyond the Solar System as an empty space that extended uniformly in all directions to immeasurable distances. He further explained through mathematical proofs and observations that all motion and dynamics in this system were explained through the single principle of universal gravitation.

However, what would come to be known as the Steady State Hypothesis did not emerge until the early 20th century. This cosmological model was inspired by a number of discoveries, as well as breakthroughs in the field of theoretical physics. These included Albert Einstein‘s Theory of General Relativity and Edwin Hubble‘s observations that the Universe is in a state of expansion.

Einstein formalized this theory by 1915 after decided to extend his theory of Special Relativity to incorporate gravity. Ultimately, this theory states that the gravitational force of matter and energy directly alters the curvature of spacetime around it. Or as famed theoretical physicist John Wheeler summarized it, “space-time tells matter how to move; matter tells space-time how to curve.”

Illustration of the depth by which Hubble imaged galaxies in prior Deep Field initiatives, in units of the Age of the Universe. Credit: NASA and A. Feild (STScI)

By 1917, theoretical calculations based on Einstein’s field equations showed that the Universe had to be in either a state of expansion or contraction. By 1929, observations made by George Lemaitre (who proposed the Big Bang Theory) and Edwin Hubble (using the 100-inch Hooker telescope at the Mount Wilson Observatory) demonstrated that the latter was the case.

Based on these revelations, a debate began by the 1930s about the possible origins and true nature of the Universe began. On one side, there were those who asserted that the Universe was finite in age and evolved over time through cooling, expansion and the formation of structures due to gravitational collapse. This theory was satirically named the “Big Bang” by Fred Hoyle, and the name stuck.

Meanwhile, the majority of astronomers at the time held to the theory that while the observable Universe is expanding, it nevertheless does not change in terms of the density of matter. In short, proponents of this theory argued that the Universe has no beginning, no end, and that matter is continuously being created over time – at a rate of one hydrogen atom per cubic meter per 100 billion years.

This theory also extended Einstein’s Cosmological Principle, aka. Cosmological Constant (CC), which Einstein proposed in 1931. According to Einstein, this force was responsible for “holding back gravity” and ensuring that the Universe remained steady, homogenous, and isotropic in terms of its large-scale structure.

Modifying this principle and extending it, members of the Steady State school of thought argued that it was the continuous creation of matter that ensured that the structure of the Universe remained the same over time. This is otherwise known as the perfect cosmological principle, which unpins the Steady State Hypothesis.

The Steady State theory became widely-known by 1948 with the publication of two papers: “A New Model for an Expanding Universe” by English astronomer Fred Hoyle, and “The Steady-State Theory and the Expanding Universe” by the British-Austrian astrophysicist and cosmologist team of Hermann Bondi and Thomas Gold.

Key Arguments and Predictions

Arguments in favor of the Steady State Hypothesis include the apparent time-scale problem raised by the observed rate of cosmic expansion (aka. the Hubble Constant or the Hubble-Lemaitre law). Based on Hubble’s observations of nearby galaxies, he calculated that the Universe was expanding at a velocity that increased systematically with distance.

This gave rise to the idea that the Universe began expanding from a much smaller volume of space. In the absence of acceleration/deceleration – 500 km/s per Megaparsec (310 mps per Mpc) – the Hubble Constant meant that all matter has been expanding for about 2 billion years – which would also be the upper age of the Universe.

What matter and antimatter might look like annihilating one another. Credit: NASA/CXC/M. Weiss

This finding was contradicted by radioactive dating, where scientists measured the rate of decay for deposits of Uranium-238 and Plutonium-205 in rock samples. Using this method, the oldest samples of rock (which were lunar in origin), were estimated to be 4.6 billion years old. Another incongruity emerged as a result of stellar evolution theory.

In short, the rate at which hydrogen is fused in the interior of stars (to create helium) yields an upper age estimate of 10 billion years for globular clusters – the oldest stars in the galaxy. What’s more, no evolution at great distances could have occurred in this model – which would mean that radio sources – aka. quasars or Active Galactic Nuclei (AGNs) – would be uniform throughout the Universe.

It would also mean that the Hubble Constant (as calculated in the early 20th century) would remain constant. The Steady-State model also predicted that the steady creation of antimatter and neutrons would result in regular annihilations and neutron decay, thus leading to the existence of a gamma-ray background and hot, x-ray emitting gas throughout the Universe.

Big Bang For The Win

However, ongoing observations during the 1950s and 1960s steadily led to a buildup of evidence against the Steady State Hypothesis. These included the discovery of bright radio sources (aka. quasars and radio galaxies) which were discovered in distant galaxies but not those closest to us – indicating that many galaxies became “radio-quiet” over time.

This single all-sky image, captured by the Planck telescope, simultaneously captured two snapshots of the CMB. Credit: ESA

By 1961, surveys of radio sources allowed for statistical analyses to be made, which ruled out the possibility that bright radio galaxies were uniformly distributed. Another major argument against the Steady State Hypothesis was the discovery of the Cosmic Microwave Background (CMB) in 1964, which the Big Bang model predicted.

Combined with the absence of a gamma-ray background and pervasive clouds of x-ray emitting gas, the Big Bang model became widely accepted by the 1960s. By the 1990s, observations with the Hubble Space Telescope and other observatories also discovered that cosmic expansion has not been consistent over time. During the last three billion years, in fact, it has been accelerating.

This has led to several refinements of the Hubble Constant. Based on data collected by the Wilkinson Microwave Anisotropy Probe (WMAP), the rate of cosmic expansion is currently estimated to be between 70 and 73.8 km/s per Mpc (43.5 to 46 mps per Mpc) with a 3% margin of error. These values are far more consistent with observations that place the age of the Universe at around 13.8 billion years.

Modern Variants

Beginning in 1993, Fred Hoyle and astrophysicists Geoffrey Burbidge and Jayant V. Narlikar began publishing a series of studies in which they proposed a new version of the Steady State Hypothesis. Known as the Quasi-Steady-State hypothesis (QSS), this variation attempted to explain cosmological phenomena that the old theory did not account for.

This model suggests that the Universe is the result of pockets of creation (aka. mini-bangs) happening over the course of many billion years. This model was modified in response to data that showed how the Universe’s rate of expansion is accelerating. Despite these modifications, the astronomical community still considers the Big Bang to be the best model for explaining all observable phenomena.

Today, this model is known as the Lambda-Cold Dark Matter (LCDM) model, which incorporates current theories about Dark Matter and Dark Energy with the Big Bang theory. In spite of that, the Steady State Hypothesis (and variants thereof) are still advocated by some astrophysicists and cosmologists. And it is not the only alternative to Big Bang Cosmology…

We have written many articles on cosmology here at Universe Today. Here’s What is the Universe, Big Bang Theory: Evolution of Our Universe, What is the Oscillating Universe Theory?, What is The Big Rip?, What is the Multiverse Theory?, What is Superstring Theory?, What is the Cosmic Microwave Background?, The Big Crunch: The End of Our Universe?, What is the Big Freeze?, and Cosmology 101: The End.

Astronomy Cast also some interesting episodes on the subject. Here’s Episode 5: The Big Bang and Cosmic Microwave Background, Episode 6: More Evidence for the Big Bang, Episode 79: How Big is the Universe?, Episode 187: History of Astronomy, Part 5: the 20th Century, and Episode 499: What is the Proposed Hubble-Lemaitre Law?.

Sources:

My Lesson Planning

Astro Tech

via Universe Today https://ift.tt/2nDj081

March 2, 2020 at 01:40PM

コメントを残す

以下に詳細を記入するか、アイコンをクリックしてログインしてください。

WordPress.com ロゴ

WordPress.com アカウントを使ってコメントしています。 ログアウト /  変更 )

Google フォト

Google アカウントを使ってコメントしています。 ログアウト /  変更 )

Twitter 画像

Twitter アカウントを使ってコメントしています。 ログアウト /  変更 )

Facebook の写真

Facebook アカウントを使ってコメントしています。 ログアウト /  変更 )

%s と連携中

WordPress.com ホスティング.

ページ先頭へ ↑

%d人のブロガーが「いいね」をつけました。