युगः

The concept of the yugá as an æra did exist from the Mántra period, as did the the concept that later ages will be more immoral than the earlier, more Divine ones. Thus we have quotes like this from the R̥g-vedá, using yugá: https://nikhilsd.com/dvq/6/

Names of four yugá-s also do occur twice in Brā́hmaṇa literature: Kr̥tá, Trétā, Dvāpára, and Káli (ĀiB 7.15.4) or Kr̥tá, Khā́rvā, Dvāpára, and Puṣyá (ṢB 5.6.5), with some of those names also occurring individually in earlier literature (hence accented) referring to sides of a die. (Khā́rvā is the vŕ̥ddhi of kharvá approximately meaning “mutilated”. Puṣyá is the name of a constellation, but I’m not sure what its meaning would be with that accent.)

Indra said to him, “The Kali is lying on the ground, the Dvâpara is hovering there; the Tretâ is getting up, but the Krita happens to walk (hither and thither). Therefore, wander, wander!”

I think it’s reasonable for a Vedist to believe in these yugá-s, but of course not in their præcise dating according to later Hindu views—and while keeping in mind that these need not be the only yugá-s, and that yugá can also refer to many other periods of time, such as yugá-s before the Gods (R̥V 10.97.1) or in post-Vedic literature a period of five years.

The Pitŕ̥-s didn’t assign an exact age to the universe, but it is at least one hundred thousand years old (TB 3.12.9.2). There is rather weak evidence that yugá in the sense of “age” was defined as being more than ten thousand years (AV 8.2.21), but no exact value is given.

I’m not aware of any Vedic reference to anything resembling cyclic time. Personally I’m neutral on the issue of cyclic vs. linear time, and believe it is unknowable in the absence of Divine revelation on the issue. I do hope that time is cyclic. The degeneracy of this age being swept away by the Gods’ cataclysmic fury followed by a return to the golden dawn of the universe would be a much more appealing future than bleak trillions of years more of this ending with a “heat death”.

One could argue that the death of the universe itself is factored into “æternity”, so references to immortality in Svargá could just mean immortality until the end of all things. I don’t think it’s strong enough evidence to confirm linearity, though it hints at it. The best-case scenario for me would be a non-linear corporeal universe and a linear Svargá, yes.