There seems to be a confusion of terms here .
1500BCE is the dating of the "arrival" of the Indo-Aryan branch of the IE family. Not the arrival of PIE people, the PIE according to Aryan invasion theory, originated in the Russian Steeps probably around 4000BCE, and then dispersed in all directions splitting up into tribes, one of those tribe the Indo-Aryans is actually the latest of the tribes emerging around 2200CE and entered India around 1500BCE from Iran, and the completed the composition of the Rig Veda by 1200BCE.
It was originally thought that Indo-Aryan people brought in the Vedas and the Vedic religion from outside of India(including the caste system) Now, it is widely accepted the Rig Veda was composed in its entirety in India, because the Rig Veda describes the flora and fauna of India, including its geography even in the early books. In other words, there is no need to posit an outside origin for the Vedic religion. The Vedic religion developed in India.
Now, this poses a problem, because the old Vedic religion is similar to the other IE religions such as the Celtic, Iranian, Slavic, Hittie, Latin, Norse and Hellenic branches. They have the same gods, the same caste-system like organisation of society and the same myths in some form. e.g. Vedic(Dyus-Pitar - sky god) Greek(Zeus) Latin(Jupiter) Celtic(Deuos-duw) Norse(Tiwaz)
See more here:
Proto-Indo-European religion - Wikipedia
So if the Vedic religion was developed in India, then this suggests the Vedic religion travelled outside of India with the IE tribes --- again pointing to India as the homeland of PIE.
It is also interesting to note the most developed version we find of the old PIE religion of the IE people is again, surprise surprise, in India. For some reason Indo-Aryans were the only branch that were able to preserve not just the PIE language the most, but also the PIE religion.
I think to an impartial, objective and rational researcher, all roads do seem to be leading back to India, So why this dogma that the Aryans were not from India but from the Russian Steeps? Again, it all based on the historical or comparative linguistics. If you want to get an idea what that is --- you basically sit down with all IE languages in front of you --- you then organise them into families (like Celtic, Latin, Hellenic, Slavic etc) and then you look at common sounds and you posit some immutable mathematical axioms of how sound shifts happen to show you which one emerged first, which one second etc so you can construct a history of how IE languages formed and split --- then somehow you also posit dates(how do you get dates from language data?) when it happened -- then it is taught as fact.
I have read several scholars within the field of linguistics itself who have seriously challenged the historical/comparative method in linguistics, even calling it a pseudoscience. I agree, because the historical method is not the scientific method, that actual sciences use like physics, chemistry and biology -- It is not based on hard repeated empirical facts, it is not tested against empirical data in the world, it is not falsifiable -- come on even Newton's laws have been falsified, so on what grounds can these historical linguists claim their science is more immutable than physics? It is as I told you earlier hubris. That has not been challenged adequately so far.
When we do actually look at the scientific data a completely different picture emerges: I will summarise some of that here, and if you want more details, I will elaborate:
1. Archaeology: There is no evidence of any invasion of the IVC -- zilch, nada. This is one of the core reasons AIT has been demoted to AMT. Now, they say the Aryans entered in small groups over several generations, but did not invade --- but still they managed to near completely supplant the IVC and turn it Aryan and even make the original Dravidian natives believe it was always Aryan for the last 10,000 years? Absurd if you ask me.
2. Geology: The Rig Veda and post-Vedic literature described a river called the 'Saraswati' which once was the mightiest river flowing from the Himalayas into the sea, it was deified as the main sustainer of the IVC civilisation and worshipped as a goddess(such as even today Hindus worship rivers as goddesses, like Ganges) There are dozens of verses in the early books of the Rig Veda describing it and eulogising it. Some of the most explicit are like 'O Saraswati, mighty river, all our settlements are on your banks' or 'O Saraswati, mighty river, may you never spurn us, or we will migrate to distant lands' In the later books of the Rig Veda the Saraswati river becomes less important and is replaced by Ganges-Yamuna(and that has remained ever since) and by the time of the Mahabharata the Saraswati river is described as starting to dry up, ending in the Thar desert.
Until only recent discoveries by space satellite imaging, this river was thought to be mythological because it could not be located. Now we know as a matter of fact this river is not mythological, it existed and it was thriving in 4000BCE, and had completely dried up by 1900BCE. This means the Aryans were already in India in 4000BCE which would make the Indo-Aryan branch by far the oldest. Hence India as the PIE homeland. There have been a few attempts to deny that this river that has been discovered is not the 'Saraswati' but the evidence is incontrovertible -- so AMT proponents have explained it away in their typical absurd ways 'The Indo-Aryans heard of the great legends and myths of the Sarwasti river from the Dravidian people, and decided to include in the Rig Veda as their main river goddess'
This is sort of like saying 'Yes we cannot detect any receivers on the dog Pluto, but that is because the receivers are invisible, but he's still plotting against us with the martians'
3. Astronomy. Not many know of the Hindu astronomical tables which have been preserved in Hindu observatories in cities like Kasi(modern day Benares) which found their way to Europe and were studied by European astronomers, including the likes of Kepler, Cassini, Newton, Brahe, Newton and Copernicus. The fact that barely anybody knows this today, is further evidence of what I said that India's role in shaping world civilisation, including up to the modern age has been marginalised in history books, and only recent research by brave scholars is starting to address the balance.
Anyway what is distinct about these Hindu tables they record the position of the planets during the Kaliyuga age at 3012BCE, which stood in direct opposition to the Mosaic calendar. At that time in Europe the Mosaic calendar was taken as religious truth, and challenging it could mean death. According to this calendar the world began in 4004BCE and it was taken as gospel truth across Europe, including with European astronomers like Newton. When they saw the Hindu tables they saw it as a huge threat to Christian civilisation because it challenged their chronology, so many European astronomers did not want to believe them. But because they were so accurate and precise they were saw as a serious challenge. There were others like Cassini who did believe them and was highly impressed with them. Bailey commented:
The motion of the stars calculated by the Hindus before some 4500 years vary not even a single minute from the tables of Cassine and Meyer (used in the 19-th century). The Indian tables give the same annual variation of the moon as the discovered by Tycho Brahe - a variation unknown to the school of Alexandria and also to the Arabs who followed the calculations of the school... "The Hindu systems of astronomy are by far the oldest and that from which the Egyptians, Greek, Romans and - even the Jews derived from the Hindus their knowledge.
The obvious implication of this the Indo-Aryans were already in India in 3012BCE at the start of the Kaliyuga era to record the position of the planets at that epoch. This tradition of recording positions of planets at auspicious occasions still continues in Hinduism today. Not only were they already in India, but they were there long enough to develop a highly sophisticated system of astronomy(which means even older origins)
So when we have such smoking gun type of evidence. Why is it rejected? Again, because it was explained away by Christian scholars at the time to maintain the Mosaic calendar, the argument was "those devious Brahmins back-calculated the position of the planets in 3012BCE to hoax that they were so ancient" It actually took a real astronomer like Playfair to show that this was impossible, because back-calculation requires knowledge of modern calculus and knowledge of gravitational constants etc, the only reasonable explanation is that these really are naked eye observations at the time.
See more here:
The Interest of European Scientists in Indian Calendar and Chronology
You see a lot of us who accept this(myth) of Aryan arrival into India in 1500BCE have no idea how this theory and date was arrived at. At the time when this theory was created India was a colonial subject and Hinduism was seen as heathen by the mainstream academia in Europe, particularly in England. Such that in older versions of Enyclopedia Brittanica, you would see "objective" descriptions like "Indian music is often rude, barbaric, inferior noise" The early indologists were all Christian missionaries(including Max Muller, William Jones) and they shared massive anxiety over the Hindu challenge to the Mosaic civilisation, which they saw as the origin of world civilisation. So they actually made it a point to revise Hindu history to make it it fit --- this is no conspiracy -- you can read their writings themselves where they first reject all Hindu dates, and then force them to fit the Mosaic account by shortening them, often by thousands of years.
I actually will submit to you there was never any "Arrival, invasion, migration" of Aryans into India. Aryans originated in India and developed this highly advanced civilisation we today call "IVC" which gave us the Dharmic religions. The IVC being the most largest and advanced civilisation in the world obviously had the power to colonise across Indo-Europe, and several tribes must have originated from India and travelled for trading, for colonisation, for tourism, cultural exchange etc This is why the oldest document we can find, other than the Vedas, is the Hittie-Mittani texts in old Semetic texts and in treaties, contain Indo-Aryan words -- indicating that there were colonies of Indo-Aryans as far as Mesopotamia (I also think there is evidence of Indo-Aryan-Semitic wars)
The Aryan arrival/invasion/migration has been maintained in history books by pure dogma. It will be gone this century, because Europe is no longer the the superpower of the world. The resurgence of India and China in the 21st century as the two most dominant powers by the end of the century will redress the balance.