I was reading this article, https://earthsky.org/space/this-date-in-science-edwin-hubble-and-the-expanding-universe . Please note that my understanding is very basic. You could find my main question toward the end. I have commented on excerpts from different sources below so that you're in a better position to understand where I'm going wrong and how I'm interpreting it. I hope it'd make it easier for you to help me. Thank you.
The following quote is taken from the mentioned article. I've boldfaced the part which originally confused and led me to post my question.
It’s said that Albert Einstein was elated to hear of Hubble’s work.
Einstein’s Theory of Relativity implied that the universe must either
be expanding or contracting. But Einstein himself rejected this notion
in favor of the accepted idea that the universe was stationary and had
always existed. When Hubble presented his evidence of the expansion of
the universe, Einstein embraced the idea. He called his adherence to
the old idea “my greatest blunder.”
I don't think that the original theory of general relativity implied expanding universe as is said above. I'd say that Einstein introduced cosmological constant as a way to counteract gravity so that the universe doesn't ultimately collapses onto itself. The three excerpts below seem to support my view.
Einstein included the cosmological constant as a term in his field
equations for general relativity because he was dissatisfied that
otherwise his equations did not allow, apparently, for a static
universe: gravity would cause a universe that was initially at dynamic
equilibrium to contract. To counteract this possibility, Einstein
added the cosmological constant.[3] However, soon after Einstein
developed his static theory, observations by Edwin Hubble indicated
that the universe appears to be expanding; this was consistent with a
cosmological solution to the original general relativity equations
that had been found by the mathematician Friedmann, working on the
Einstein equations of general relativity. Einstein reportedly referred
to his failure to accept the validation of his equations—when they had
predicted the expansion of the universe in theory, before it was
demonstrated in observation of the cosmological redshift— as his
"biggest blunder".[10]In fact, adding the cosmological constant to Einstein's equations does
not lead to a static universe at equilibrium because the equilibrium
is unstable: if the universe expands slightly, then the expansion
releases vacuum energy, which causes yet more expansion. Likewise, a
universe that contracts slightly will continue contracting.[11]
Source 1: https://en.wikipedia.org/wiki/Cosmological_constant#History
The cosmological constant was first proposed by Einstein as a
mechanism to obtain a solution of the gravitational field equation
that would lead to a static universe, effectively using dark energy to
balance gravity.[16] Einstein gave the cosmological constant the
symbol Λ (capital lambda)….The mechanism was an example of fine-tuning, and it was later realized
that Einstein's static universe would not be stable: local
inhomogeneities would ultimately lead to either the runaway expansion
or contraction of the universe. The equilibrium is unstable: if the
universe expands slightly, then the expansion releases vacuum energy,
which causes yet more expansion. Likewise, a universe which contracts
slightly will continue contracting. These sorts of disturbances are
inevitable, due to the uneven distribution of matter throughout the
universe. Further, observations made by Edwin Hubble in 1929 showed
that the universe appears to be expanding and not static at all.
Einstein reportedly referred to his failure to predict the idea of a
dynamic universe, in contrast to a static universe, as his greatest
blunder.[19]
Source 2: https://en.wikipedia.org/wiki/Dark_energy#History_of_discovery_and_previous_speculation
In 1912, Vesto Slipher discovered that light from remote galaxies was
redshifted,[8][9] which was later interpreted as galaxies receding
from the Earth. In 1922, Alexander Friedmann used Einstein field
equations to provide theoretical evidence that the universe is
expanding.[10] In 1927, Georges Lemaître independently reached a
similar conclusion to Friedmann on a theoretical basis, and also
presented the first observational evidence for a linear relationship
between distance to galaxies and their recessional velocity.[11] Edwin
Hubble observationally confirmed Lemaître's findings two years
later.[12] Assuming the cosmological principle, these findings would
imply that all galaxies are moving away from each other.Based on large quantities of experimental observation and theoretical
work, the scientific consensus is that space itself is expanding, and
that it expanded very rapidly within the first fraction of a second
after the Big Bang. This kind of expansion is known as "metric
expansion". In mathematics and physics, a "metric" means a measure of
distance, and the term implies that the sense of distance within the
universe is itself changing.
Source 3: https://en.wikipedia.org/wiki/Expansion_of_the_universe#History
Further, I don't think it was a blunder on Einstein's part to remove the constant, as is stated in the excerpt below from Source 4. Einstein had used the cosmological constant to balance the attractive force gravity so that the end result was static universe as the static model was preferred at that time. It's not that if Einstein hadn't removed the constant, it'd have resulted into expanding universe. In my opinion, removing it would have rather resulted into collapsing universe which Einstein had initially tried to prevent.
Albert Einstein invented the cosmological constant as part of his
theory of gravity, known as General Relativity.Einstein’s equations showed that the gravity of all the matter in the
universe would exert a strong pull, pulling all the stars and galaxies
toward each other and eventually causing the universe to collapse. At
the time, though, astronomers believed that the universe was static –
that it was neither expanding nor contracting. To counteract this
problem, Einstein added another term to his equations, called the
cosmological constant, to balance the inward pull of gravity.Within about a decade, though, astronomer Edwin Hubble discovered that
the universe is expanding. Einstein discarded the cosmological
constant, calling it his greatest scientific blunder.When dark energy was discovered, though, many physicists began to
think that Einstein’s only blunder was in removing the constant. This
"repulsive" force could begin to explain the acceleration of the
universe. In other words, it might be the dark energy.
Source 4: http://hetdex.org/dark_energy/what_is_it/vacuum_energy.html
MAIN QUESTION:
Dark energy was discovered in the 1990 as stated in Source 6 and I also think, as in Source 5, that dark energy is quite different from the cosmological constant. Around 1930 Einstein was convinced that the universe was expanding, did he entirely remove the constant from his equations or did he change its value so that the equations predict expanding universe? What was thought at that time as the cause of expansion? What was there in the space which was pushing it apart? How did Friedmann in 1922 reach the conclusion that the universe was expanding using the original equations of general relativity laid down by Einstein? What was Friedmann's motivation? I found a similar question here but the answer is mathematical heavy and I couldn't make any sense of it at all.
Basilakos and Sola instead suggest that dark energy is a type of
dynamic quantum vacuum energy — something different than Einstein's
cosmological constant, which describes a static vacuum energy density
and is another possible explanation of dark energy's nature.
Source 5: https://www.space.com/25238-dark-energy-quantum-vacuum-theory.html
Dark energy is even more mysterious, and its discovery in the 1990s
was a complete shock to scientists. Previously, physicists had assumed
that the attractive force of gravity would slow down the expansion of
the universe over time. But when two independent teams tried to
measure the rate of deceleration, they found that the expansion was
actually speeding up. One scientist likened the finding to throwing a
set of keys up in the air expecting them to fall back down-only to see
them fly straight up toward the ceiling.
Source 6: https://www.nationalgeographic.com/science/space/dark-matter/
It has become a popular belief that Albert Einstein abandoned his
static universe when, on a visit to Pasadena in January and February
1931, Edwin Hubble showed him the redshifted nebular spectra and
convinced him that the universe was expanding, and the cosmological
constant was superfluous.
Source 7: https://arxiv.org/ftp/arxiv/papers/1311/1311.2763.pdf (Einstein’s conversion from his static to an expanding universe)
Helpful links:
1: What's the difference between "dark energy" and "vacuum energy"?
2: https://www.youtube.com/watch?v=g20JZ2HNZaw
EDIT #1: (added on 12-25-2020, 12:26 PM UTC)
Earlier at the beginning on this post I said, "I don't think that the original theory of general relativity implied expanding universe as is said above. I'd say that Einstein introduced cosmological constant as a way to counteract gravity so that the universe doesn't ultimately collapses onto itself."
I think I should add more to it. I was watching the documentary "The Fabric of the Cosmos: What Is Space?" presented by Brian Greene.
The following is said around 40:33 – 43:10.
and the discovery of dark energy held another surprise the idea that
the universe contains such an ingredient had actually been cooked up
eighty years earlier I'll let you in on a little secret although he
didn't call it dark energy long ago Albert Einstein predicted that
space itself could exert a force that would drive galaxies apart you
see shortly after discovering his general theory of relativity his
theory of gravity Einstein found that according to the mathematics
the universe would either be expanding or contracting but it couldn't
hover at a fixed size this was puzzling because before they knew about
the Big Bang most scientists including Einstein pictured the universe
as static eternal and unchanging when Einstein's equations suggested
an expanding or contracting universe not the static universe everyone
believed in he had a problem so Einstein went back to his equations
and modified them to allow for kind of anti-gravity that would infuse
space with an outward push counteracting usual inward pull of gravity
allowing the universe to stand still he called the modification the
cosmological constant adding the cosmological constant rescued his
equations but the truth is Einstein had no idea if his outward push or
anti-gravity really existed the introduction of the cosmological
constant by Einstein was not a very elegant solution to try to find
what he was looking for stationary universe it achieves this effect of
anti-gravity it says that gravity sometimes can behave in such a way
is not to pull things together but to push things apart like the clash
of two titans the cosmological constant and the pull of ordinary
matter could hold the universe in check and keep it static but about a
dozen years later the astronomer Edwin Hubble discovered the universe
is not static it's expanding due to the explosive force of the Big
Bang fourteen billion years ago that meant Einsteins original
equations no longer had to be altered so suddenly the need for a
cosmological constant went right out the window.
It's understandable that the gravity could pull everything inward and make the universe collapse but, looking at it historically, it's not clear what was there in the space which would push everything outward resulting into expanding universe. Around 1920s it was not thought that space had any inherent energy which would push the matter such as galaxies apart. Yes, Einstein's space-time was dynamic which could bend, ripple, twist, etc. but it didn't have any inherent energy. By the way, if it had its own energy, it would also result into gravity. So, what was there in Einstein's equations which predicted or implied expanding universe? I think the answer lies not in the math. Also note that Big Bang theory was formulated around 1931.
Einstein was trying to come up with a cosmology model for the universe using his gravitational field equations and it required few assumptions to start with; I think the term is initial conditions. He could either have assumed that, for some unknown reason, the universe is expanding which also counteracted the attractive gravity, or the universe was static and this would require some fixed outward force to counteract the gravity. Einstein went for the static universe because this was the most prevalent view of the universe at that time though, as I've read, mathematically, not very stable. In short, it wasn't that the math itself was implying something, it was more about the assumptions used to come up with the model. Please see the excerpt below.
In the early 20th century the common worldview held that the universe
is static — more or less the same throughout eternity. Einstein
expressed the general opinion in 1917 after de Sitter produced
equations that could describe a universe that was expanding, a
universe with a beginning. Einstein wrote him that "This circumstance
irritates me." In another letter, Einstein added: "To admit such
possibilities seems senseless."In his gravitational field equations, Einstein was just then providing
a compact mathematical tool that could describe the general
configuration of matter and space taking the universe as a whole. The
peculiar curvature of space predicted in the equations was quickly
endorsed in famous experiments, and by the early 1920s most leading
scientists agreed that Einstein's field equations could make a
foundation for cosmology. The only problem was that finding a solution
to these simple equations — that is, producing a model of the universe
— was a mathematical nightmare.
…Einstein met with de Sitter in neutral Holland. Stimulating and
criticizing each other, they produced two cosmological models, two
different solutions to the field equations. But both models seemed to
need special adjustments.Einstein's first try at a model likewise could not contain matter and
be stable. For the equations showed that if the universe was static at
the outset, the gravitational attraction of the matter would make it
all collapse in upon itself. That seemed ridiculous, for there was no
reason to suppose that space was so unstable.Einstein found he could stabilize his model by adding a simple
constant term to the equations. If this constant was not zero, the
model would not have to collapse under its own gravity. This
"cosmological constant," Einstein admitted, was only "a hypothetical
term." It was "not required by the theory as such nor did it seem
natural from a theoretical point of view." In fact, "The term is
necessary only for the purpose of making possible a quasi-static
distribution of matter."
Source: https://history.aip.org/exhibits/cosmology/ideas/expanding.htm
Best Answer
I'll try and briefly run through some points (without mathematical detail) to see if this clears up any of your questions
• You seem to have misunderstood the 'blunder' part: the 'blunder' wasn't removing the cosmological constant, but adding it to his equations (in an ad-hoc way, at the time) in the first place.
• Today, dark energy isn't 'different' from the cosmological constant - the CC is just a possible (and the simplest) way to describe dark energy. (It's also the one that works best, despite the surrounding theoretical issues.)
• Einstein removed the CC because it was no longer needed for a static universe (and the other associated problems regarding stability, which you quoted).
Just to be clear, neither a cosmological constant nor any type of dark energy is needed for an expanding universe, but is needed for accelerated expansion. The universe was already expanding from the big bang. A universe where the CC is zero still expands.