Are astronomy and cosmology same? If not, what are the differences? And which one is greater in terms of subject area?
(Physical) cosmology analyzes the universe on its largest scale. Rather than studying individual objects in the universe, it focuses on the overall universe's properties (its shape, composition, age, etc.) and its evolution (how it originated, how it's changing, and how it will end).
"Astronomy" is a less precisely defined term. In its broadest sense, it's simply the study of outer space, but it can take on more specific meanings depending on the context and who you ask. Some people will say astronomy solely analyzes the universe observationally, whereas "astrophysics" does so with physics; others consider "astronomy" and "astrophysics" to be interchangeable. Likewise, some will say astronomy solely studies the contents in the universe, making it distinct from cosmology's focus on the overall universe; others consider astronomy to be a more broad field encompassing cosmology.
No, they are not the same. Astronomy basically represents the "old science" about the universe: predicting motion of the stars and planets, eclipses, and so on. Look on astronomy as something Greeks would do (but it is not equal to their astronomy; many other modern methods have evolved since then, like neutrino, X-ray or gravitational-wave astronomy).
Cosmology views the universe as a whole: its expanding, objects on large scale, laws, etc. One branch of it is also some sort of philosophy: why do we exist, how did everything start ... It is called philosophy of cosmology. Cosmology is thus on higher scale, while astronomy is on lower scale.
Cosmology is a subfield of astronomy. Cosmology deals with very big things (even by astronomical standards). Astronomy on the other hand is an umbrella term that includes lots of smaller-scale stuff.
Here are some topics in cosmology:
- Hubble expansion
- Big Bang nucleosynthesis
- Cosmic Inflation
- Cosmic Microwave Background
Here are some topics in astronomy, but not cosmology:
- Extrasolar planets
- Extraterrestrial life
- Stellar / Neutron star / Black hole collisions
- Galaxy dynamics (e.g. how do galaxies evolve?)
- Stellar nucleosynthesis
Here are some topics that overlap between the two fields:
- Dark matter
- Dark energy (although this is significantly more in cosmology than astronomy - at non-cosmological distances, dark energy is seldom relevant)
- Structure formation (although the biggest structures such as galaxy clusters are within cosmology; the smaller ones like galaxies are no)
- Redshift (by $z = 1$, we are firmly in the regime of cosmology, but smaller redshifts are relevant to astronomers too)
- Gravitational lensing
In view of the various (and varying) answers above (3 at the current posting: Sir Cumference, Allure, User123), the relevance of this annoyingly challenging question is established. Nevertheless, the posted answers seem to contradict themselves, if not containing contradictions individually. Even if there seems to be agreement that Cosmology and Astronomy are different, there doesn’t seem to be a clear consensus on how they differ and are related. There may be a vague convergence on the notion that Cosmology is characterized as more holistic (eg “whole”, “bigger/higher”) than Astronomy. One answer even goes as far as saying that Astronomy is “less precisely defined” (implying perhaps that Astronomy can encompass Cosmology, “depending on who you are talking to”).
So let’s assume that the author of this question is looking for clues to decide on his academic future (say which department to enroll in, in a prestigious university that puts Astronomy and Cosmology in different departments). Let’s ask ourselves whether Astronomy SE has answered satisfactorily here? I think we have not completely yet. So let me try to throw in additional arguments and clues.
Are Astronomy and Cosmology the same?
No... But, there are commonalities (which make things confusingly intriguing indeed!)
The ancient Greek invented these two concepts. The following site gives a brief intro
Astronomy in ancient Greek means “laws of the (visible) stars”. Today, we extend this definition to:
The science of the celestial bodies and the Universe, dealing especially with the positions, dimensions, distribution, motion, chemical composition, energy, and evolution of celestial bodies and phenomena. (http://dictionary.obspm.fr/index.php?showAll=1&&search=A&&formSearchTextfield=&&page=41)
Cosmos in ancient Greek means “order (of everything)”. Today, we use it as:
The science of the origin, structure, and evolution of the Universe including the origin of galaxies, the chemical elements, and matter. (http://dictionary.obspm.fr/index.php?showAll=1&&search=C&&formSearchTextfield=&&page=57)
So in ancient Greek, Astronomy is the science that look for the order (the laws governing) in the observations (of the Celestial Sphere). Cosmos designates the abstract whole thing that would be governed by the discovered set of laws. If you know the laws, by definition you can predict the evolution of anything in Cosmos (that Cosmos), visible or not, detectable or not yet detectable. If you don’t know the laws, you can’t predict and it would be Chaos, not Cosmos that is observed.
This begs the question: Has these 2 fields merged today. Put differently, can I specialize in one field, but not the other?
The answer is no, they haven’t merged (yet). As an example, Stephen Hawkins has never been described as an astronomer. Wiki describes Carl Sagan as both an astronomer and a cosmologist (among other qualifications), which seems to indicate that the two fields are getting closer, or at least very closely related. It would be difficult to specialize in one while having only casual knowledge of the other.
Another clue to support this answer that Astronomy and Cosmology are different fields of researches, but increasingly related, is that last year’s Nobel prize in Physics was rewarded to 3 scientists:
(Sir) Roger Penrose, a mathematician and cosmologist. Andrea Ghez, an astronomer. Reinhard Genzel, an astrophysicist.
As a closing note, we use “astronaut” and “cosmonaut” interchangeably (taikonaut for the Chinese). Which seems to defeat the answer that the 2 terms are different. In fact, it may indicate that, for these _nauts, the difference is irrelevant. All they need is to master the overlap of the 2 sciences.